Add ConditionTrait.evaluate() (#909)

Add `ConditionTrait.evaluate()` so that a condition can be evaluated
independent of a `Test` object.

### Motivation:

Currently, the only way a `ConditionTrait` is evaluated is inside the
`prepare(for:)` method. This makes it difficult and awkward for
third-party libraries to utilize these traits because evaluating a
condition would require creating a dummy `Test` to pass to that method.

### Modifications:

Add `ConditionTrait.evaluate()`, and `ConditionTrait.Evaluation` enum
for the return value.

### Result:

Public API allows for evaluating a `ConditionTrait` in any context.

### 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: Uncommon

Sources/Testing/Traits/ConditionTrait.swift

@@ -62,16 +62,27 @@ public struct ConditionTrait: TestTrait, SuiteTrait {
 
   /// The source location where this trait is specified.
   public var sourceLocation: SourceLocation
-
-  public func prepare(for test: Test) async throws {
-    let result = switch kind {
+  
+  /// Evaluate this instance's underlying condition.
+  ///
+  /// - Returns: The result of evaluating this instance's underlying condition.
+  ///
+  /// The evaluation is performed each time this function is called, and is not
+  /// cached.
+  @_spi(Experimental)
+  public func evaluate() async throws -> Bool {
+    switch kind {
     case let .conditional(condition):
       try await condition()
     case let .unconditional(unconditionalValue):
       unconditionalValue
     }
+  }
+
+  public func prepare(for test: Test) async throws {
+    let isEnabled = try await evaluate()
 
-    if !result {
+    if !isEnabled {
       // We don't need to consider including a backtrace here because it will
       // primarily contain frames in the testing library, not user code. If an
       // error was thrown by a condition evaluated above, the caller _should_

Tests/TestingTests/Traits/ConditionTraitTests.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-@testable import Testing
+@testable @_spi(Experimental) import Testing
 
 @Suite("Condition Trait Tests", .tags(.traitRelated))
 struct ConditionTraitTests {
@@ -41,4 +41,22 @@ struct ConditionTraitTests {
     .disabled(if: false)
   )
   func disabledTraitIf() throws {}
+  
+  @Test
+  func evaluateCondition() async throws {
+    let trueUnconditional = ConditionTrait(kind: .unconditional(true), comments: [], sourceLocation: #_sourceLocation)
+    let falseUnconditional = ConditionTrait.disabled()
+    let enabledTrue = ConditionTrait.enabled(if: true)
+    let enabledFalse = ConditionTrait.enabled(if: false)
+    var result: Bool
+    
+    result = try await trueUnconditional.evaluate()
+    #expect(result)
+    result = try await falseUnconditional.evaluate()
+    #expect(!result)
+    result = try await enabledTrue.evaluate()
+    #expect(result)
+    result = try await enabledFalse.evaluate()
+    #expect(!result)
+  }
 }