Improve collection assertions

Asserting with Assert.Empty / Assert.NotEmpty gives much better error messages when the assertion fails.

Before:

```
Assert.True() Failure
Expected: True
Actual:   False
   at XmlSchemaClassGenerator.Tests.Compiler.Compile(String name, String[] contents) in XmlSchemaClassGenerator/XmlSchemaClassGenerator.Tests/Compiler.cs:line 152
```

After:

```
Assert.Empty() Failure
Collection: [(11144,6): error CS0579: Duplicate 'System.CodeDom.Compiler.GeneratedCodeAttribute' attribute, (11145,6): error CS0579: Duplicate 'System.SerializableAttribute' attribute, (11147,6): error CS0579: Duplicate 'System.Xml.Serialization.XmlRootAttribute' attribute, (10023,26): error CS0146: Circular base class dependency involving 'User' and 'User']
   at XmlSchemaClassGenerator.Tests.Compiler.Compile(String name, String[] contents) in XmlSchemaClassGenerator/XmlSchemaClassGenerator.Tests/Compiler.cs:line 152
```

XmlSchemaClassGenerator.Tests/Compiler.cs

@@ -1,4 +1,4 @@
-using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Emit;
 using System;
@@ -149,11 +149,9 @@ public static Assembly Compile(string name, params string[] contents)
  .WithOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));
  var result = Compiler.GenerateAssembly(compilation);
 
+ Assert.Empty(result.Result.Diagnostics.Where(d => d.Severity == DiagnosticSeverity.Error));
+ Assert.Empty(result.Result.Diagnostics.Where(d => d.Severity == DiagnosticSeverity.Warning));
  Assert.True(result.Result.Success);
- var errors = result.Result.Diagnostics.Where(d => d.Severity == DiagnosticSeverity.Error).ToList();
- Assert.False(errors.Any(), string.Join("\n", errors.Select(e => e.GetMessage())));
- var warnings = result.Result.Diagnostics.Where(d => d.Severity == DiagnosticSeverity.Warning).ToList();
- Assert.False(warnings.Any(), string.Join("\n", errors.Select(w => w.GetMessage())));
  Assert.NotNull(result.Assembly);
 
  Assemblies[name] = result.Assembly;

XmlSchemaClassGenerator.Tests/XmlTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.CodeDom;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
@@ -157,7 +157,7 @@ public void TestListWithPrivatePropertySetters()
  var iListType = typeof(Collection<>);
  var collectionPropertyInfos = myClassType.GetProperties().Where(p => p.PropertyType.IsGenericType && iListType.IsAssignableFrom(p.PropertyType.GetGenericTypeDefinition())).OrderBy(p=>p.Name).ToList();
  var publicCollectionPropertyInfos = collectionPropertyInfos.Where(p => p.SetMethod.IsPrivate).OrderBy(p=>p.Name).ToList();
- Assert.True(collectionPropertyInfos.Count > 0);
+ Assert.NotEmpty(collectionPropertyInfos);
  Assert.Equal(collectionPropertyInfos, publicCollectionPropertyInfos);
 
  var myClassInstance = Activator.CreateInstance(myClassType);
@@ -189,7 +189,7 @@ public void TestListWithPublicPropertySetters()
  var iListType = typeof(Collection<>);
  var collectionPropertyInfos = myClassType.GetProperties().Where(p => p.PropertyType.IsGenericType && iListType.IsAssignableFrom(p.PropertyType.GetGenericTypeDefinition())).OrderBy(p=>p.Name).ToList();
  var publicCollectionPropertyInfos = collectionPropertyInfos.Where(p => p.SetMethod.IsPublic).OrderBy(p=>p.Name).ToList();
- Assert.True(collectionPropertyInfos.Count > 0);
+ Assert.NotEmpty(collectionPropertyInfos);
  Assert.Equal(collectionPropertyInfos, publicCollectionPropertyInfos);
 
  var myClassInstance = Activator.CreateInstance(myClassType);
@@ -221,7 +221,7 @@ public void TestListWithPublicPropertySettersWithoutConstructors()
  var iListType = typeof(Collection<>);
  var collectionPropertyInfos = myClassType.GetProperties().Where(p => p.PropertyType.IsGenericType && iListType.IsAssignableFrom(p.PropertyType.GetGenericTypeDefinition())).OrderBy(p=>p.Name).ToList();
  var publicCollectionPropertyInfos = collectionPropertyInfos.Where(p => p.SetMethod.IsPublic).OrderBy(p=>p.Name).ToList();
- Assert.True(collectionPropertyInfos.Count > 0);
+ Assert.NotEmpty(collectionPropertyInfos);
  Assert.Equal(collectionPropertyInfos, publicCollectionPropertyInfos);
  var myClassInstance = Activator.CreateInstance(myClassType);
  foreach (var collectionPropertyInfo in publicCollectionPropertyInfos)
@@ -253,7 +253,7 @@ public void TestListWithPublicPropertySettersWithoutConstructorsSpecified()
  var iListType = typeof(Collection<>);
  var collectionPropertyInfos = myClassType.GetProperties().Where(p => p.PropertyType.IsGenericType && iListType.IsAssignableFrom(p.PropertyType.GetGenericTypeDefinition())).OrderBy(p => p.Name).ToList();
  var publicCollectionPropertyInfos = collectionPropertyInfos.Where(p => p.SetMethod.IsPublic).OrderBy(p => p.Name).ToList();
- Assert.True(collectionPropertyInfos.Count > 0);
+ Assert.NotEmpty(collectionPropertyInfos);
  Assert.Equal(collectionPropertyInfos, publicCollectionPropertyInfos);
  var myClassInstance = Activator.CreateInstance(myClassType);