Tests: Fix always succeeding XCTAssertAsyncNoThrow (#8026)

The autoclosure expression `{ throw error }` just creates a closure that
won't be executed. It leads to the test always succeeding even if the
`expression` throws an error.

Fortunately, I didn't find any issue to enable the assertion except for
a single case
(ab1e784)

Fixtures/Miscellaneous/CheckTestLibraryEnvironmentVariable/Tests/CheckTestLibraryEnvironmentVariableTests/CheckTestLibraryEnvironmentVariableTests.swift

@@ -2,7 +2,7 @@ import XCTest
 
 final class CheckTestLibraryEnvironmentVariableTests: XCTestCase {
     func testEnviromentVariable() throws {
-        let envvar = ProcessInfo.processInfo.environment["SWIFT_PM_TEST_LIBRARY"]
-        XCTAssertEqual(envvar, "XCTest")
+        let envvar = ProcessInfo.processInfo.environment["SWIFT_TESTING_ENABLED"]
+        XCTAssertEqual(envvar, "0")
     }
 }

Sources/_InternalTestSupport/XCTAssertHelpers.swift

@@ -74,7 +74,7 @@ package func XCTAssertAsyncNoThrow<T>(
     do {
         _ = try await expression()
     } catch {
-        XCTAssertNoThrow({ throw error }, message(), file: file, line: line)
+        XCTAssertNoThrow(try { throw error }(), message(), file: file, line: line)
     }
 }
 

Tests/CommandsTests/TestCommandTests.swift

@@ -327,11 +327,14 @@ final class TestCommandTests: CommandsTestCase {
     }
 #endif
 
+#if os(macOS)
+    // "SWIFT_TESTING_ENABLED" is set only on macOS, skip the check on other platforms.
     func testLibraryEnvironmentVariable() async throws {
         try await fixture(name: "Miscellaneous/CheckTestLibraryEnvironmentVariable") { fixturePath in
             await XCTAssertAsyncNoThrow(try await SwiftPM.Test.execute(packagePath: fixturePath))
         }
     }
+#endif
 
     func testXCTestOnlyDoesNotLogAboutNoMatchingTests() async throws {
         try await fixture(name: "Miscellaneous/TestDiscovery/Simple") { fixturePath in