Add test for new trait filtering behavior

Author: gmedori

Tests/TestingTests/Test.TraitTests.swift

@@ -0,0 +1,52 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+@testable @_spi(Experimental) @_spi(ForToolsIntegrationOnly) import Testing
+
+/// A trait that only applies to suites
+private struct SuiteOnlyTrait: SuiteTrait {}
+
+/// A trait that only applies to test functions
+private struct TestOnlyTrait: TestTrait {}
+
+/// A trait that applies to both
+private struct TestAndSuiteTrait: SuiteTrait, TestTrait {}
+
+/// A stub test suite.
+@Suite
+private struct DemoTestSuite {}
+
+@Suite("Test.Trait Tests")
+struct Test_TraitTests {
+  @Test("Setting traits on a suite filters out non-suite traits", .tags(.traitRelated))
+  func filterTraitsOnSuites() async throws {
+    let mixedTraits: [any Trait] = [SuiteOnlyTrait(), TestOnlyTrait(), TestAndSuiteTrait()]
+    var suite = try #require(await test(for: DemoTestSuite.self))
+
+    // Setting the traits property should filter out non-suite traits
+    suite.traits = mixedTraits
+    #expect(suite.traits.count == 2)
+    #expect(suite.traits[0] is SuiteOnlyTrait)
+    #expect(suite.traits[1] is TestAndSuiteTrait)
+  }
+
+  @Test("Setting traits on a test filters out non-test traits", .tags(.traitRelated))
+  func filterTraitsOnTests() async throws {
+    let mixedTraits: [any Trait] = [SuiteOnlyTrait(), TestOnlyTrait(), TestAndSuiteTrait()]
+    var test = Test {}
+
+    // Setting the traits property should filter out non-test traits
+    test.traits = mixedTraits
+    #expect(test.traits.count == 2)
+    #expect(test.traits[0] is TestOnlyTrait)
+    #expect(test.traits[1] is TestAndSuiteTrait)
+  }
+
+}