Fix issues building when using Xcode Command Line Tools (no XCTest.)

This PR removes some stray dependencies on XCTest that prevent building the
testing library and its test target when XCTest is not available on Darwin. This
can occur if the Xcode Command Line Tools are installed instead of the full IDE.

Automated testing for this change is difficult because it relies on a build
environment that is not supported in CI (namely the presence of the CL tools but
not Xcode nor XCTest.framework.) I have manually tested the change against
swift-testing's own test target.

For more information about this change, see swiftlang/swift-package-manager#7426

Resolves rdar://125371899.

Author: grynspan

Sources/Testing/Running/XCTestScaffold.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if !SWT_NO_XCTEST_SCAFFOLDING
+#if !SWT_NO_XCTEST_SCAFFOLDING && canImport(XCTest)
 private import TestingInternals
 public import XCTest
 
@@ -25,19 +25,6 @@ extension XCTSourceCodeContext {
     self.init(callStackAddresses: addresses, location: sourceLocation)
   }
 }
-
-/// An error that is reported by ``XCTestScaffold`` when a test times out.
-///
-/// This type is not part of the public interface of the testing library.
-struct TimeoutError: Error, CustomStringConvertible {
-  /// The time limit exceeded by the test that timed out.
-  var timeLimit: TimeValue
-
-  var description: String {
-    "Timed out after \(timeLimit) seconds."
-  }
-}
-
 extension XCTIssue {
   init(_ issue: Issue, processLaunchedByXcode: Bool) {
     var error = issue.error

Sources/Testing/Traits/TimeLimitTrait.swift

@@ -108,6 +108,18 @@ extension Test {
 
 // MARK: -
 
+/// An error that is reported by ``XCTestScaffold`` when a test times out.
+///
+/// This type is not part of the public interface of the testing library.
+struct TimeoutError: Error, CustomStringConvertible {
+  /// The time limit exceeded by the test that timed out.
+  var timeLimit: TimeValue
+
+  var description: String {
+    "Timed out after \(timeLimit) seconds."
+  }
+}
+
 #if !SWT_NO_UNSTRUCTURED_TASKS
 /// Invoke a function with a timeout.
 ///

Tests/TestingMacrosTests/TestSupport/Scaffolding.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if !SWT_NO_XCTEST_SCAFFOLDING && !SWIFT_PM_SUPPORTS_SWIFT_TESTING
+#if !SWT_NO_XCTEST_SCAFFOLDING && !SWIFT_PM_SUPPORTS_SWIFT_TESTING && canImport(XCTest)
 import XCTest
 import Testing
 

Tests/TestingTests/IssueTests.swift

@@ -10,6 +10,7 @@
 
 @testable @_spi(Experimental) @_spi(ForToolsIntegrationOnly) import Testing
 private import TestingInternals
+import Foundation
 
 #if canImport(XCTest)
 import XCTest

Tests/TestingTests/ObjCInteropTests.swift

@@ -8,6 +8,7 @@
  See https://swift.org/CONTRIBUTORS.txt for Swift project authors
  */
 
+#if canImport(XCTest)
 import XCTest
 @testable @_spi(ForToolsIntegrationOnly) import Testing
 
@@ -106,3 +107,4 @@ struct ObjCAndXCTestInteropTests {
     }
   }
 }
+#endif

Tests/TestingTests/RunnerTests.swift

@@ -10,6 +10,7 @@
 
 #if canImport(XCTest)
 import XCTest
+#endif
 @testable @_spi(Experimental) @_spi(ForToolsIntegrationOnly) import Testing
 
 struct MyError: Error, Equatable {
@@ -35,6 +36,7 @@ private let randomNumber = Int.random(in: 0 ..< .max)
   throw MyParameterizedError(index: i)
 }
 
+#if canImport(XCTest)
 @Suite(.hidden, .disabled())
 struct NeverRunTests {
   private static var someCondition: Bool {

Tests/TestingTests/TestSupport/Scaffolding.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if !SWT_NO_XCTEST_SCAFFOLDING && !SWIFT_PM_SUPPORTS_SWIFT_TESTING
+#if !SWT_NO_XCTEST_SCAFFOLDING && !SWIFT_PM_SUPPORTS_SWIFT_TESTING && canImport(XCTest)
 import XCTest
 import Testing