Add ui test for useless_concat lint

tests/ui/useless_concat.fixed

@@ -0,0 +1,10 @@
+#![warn(clippy::useless_concat)]
+#![allow(clippy::print_literal)]
+
+fn main() {
+    let x = ""; //~ useless_concat
+    let x = "a"; //~ useless_concat
+    println!("b: {}", "a"); //~ useless_concat
+    // Should not lint.
+    let x = concat!("a", "b");
+}

tests/ui/useless_concat.rs

@@ -0,0 +1,10 @@
+#![warn(clippy::useless_concat)]
+#![allow(clippy::print_literal)]
+
+fn main() {
+    let x = concat!(); //~ useless_concat
+    let x = concat!("a"); //~ useless_concat
+    println!("b: {}", concat!("a")); //~ useless_concat
+    // Should not lint.
+    let x = concat!("a", "b");
+}

tests/ui/useless_concat.stderr

@@ -0,0 +1,23 @@
+error: unneeded use of `concat!` macro
+  --> tests/ui/useless_concat.rs:5:13
+   |
+LL |     let x = concat!();
+   |             ^^^^^^^^^ help: replace with: `""`
+   |
+   = note: `-D clippy::useless-concat` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::useless_concat)]`
+
+error: unneeded use of `concat!` macro
+  --> tests/ui/useless_concat.rs:6:13
+   |
+LL |     let x = concat!("a");
+   |             ^^^^^^^^^^^^ help: replace with: `"a"`
+
+error: unneeded use of `concat!` macro
+  --> tests/ui/useless_concat.rs:7:23
+   |
+LL |     println!("b: {}", concat!("a"));
+   |                       ^^^^^^^^^^^^ help: replace with: `"a"`
+
+error: aborting due to 3 previous errors
+