Disallow the @Test attribute on operator declarations. (#1205)

This PR prevents `@Test` from being applied to an operator declaration
such as:

```swift
@test(arguments: ...)
static func +(lhs: A, rhs: B) { ... }
```

Now, the following error will be emitted by the compiler:

> 🛑 Attribute 'Test' cannot be applied to an operator

Previously, applying `@Test` to an operator produced undefined/unstable
effects.

Resolves #1204.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift

@@ -34,6 +34,16 @@ extension FunctionDeclSyntax {
       .contains(.keyword(.nonisolated))
   }
 
+  /// Whether or not this function declares an operator.
+  var isOperator: Bool {
+    switch name.tokenKind {
+    case .binaryOperator, .prefixOperator, .postfixOperator:
+      true
+    default:
+      false
+    }
+  }
+
   /// The name of this function including parentheses, parameter labels, and
   /// colons.
   var completeName: DeclReferenceExprSyntax {

Sources/TestingMacros/Support/DiagnosticMessage.swift

@@ -93,7 +93,11 @@ struct DiagnosticMessage: SwiftDiagnostics.DiagnosticMessage {
     let result: (value: String, article: String)
     switch node.kind {
     case .functionDecl:
-      result = ("function", "a")
+      if node.cast(FunctionDeclSyntax.self).isOperator {
+        result = ("operator", "an")
+      } else {
+        result = ("function", "a")
+      }
     case .classDecl:
       result = ("class", "a")
     case .structDecl:

Sources/TestingMacros/TestDeclarationMacro.swift

@@ -61,7 +61,7 @@ public struct TestDeclarationMacro: PeerMacro, Sendable {
     }
 
     // The @Test attribute is only supported on function declarations.
-    guard let function = declaration.as(FunctionDeclSyntax.self) else {
+    guard let function = declaration.as(FunctionDeclSyntax.self), !function.isOperator else {
       diagnostics.append(.attributeNotSupported(testAttribute, on: declaration))
       return false
     }

Tests/TestingMacrosTests/TestDeclarationMacroTests.swift

@@ -67,6 +67,8 @@ struct TestDeclarationMacroTests {
         "Attribute 'Test' cannot be applied to a structure",
       "@Test enum E {}":
         "Attribute 'Test' cannot be applied to an enumeration",
+      "@Test func +() {}":
+        "Attribute 'Test' cannot be applied to an operator",
 
       // Availability
       "@available(*, unavailable) @Suite struct S {}":