Fix a failing time-sensitive test. (#190)

One of the tests for our `.timeLimit()` trait uses `Clock.measure {}` to determine how long it takes to execute, but that's unsafe because it will include time spent waiting for the Swift runtime and the OS to schedule the child task in which the test runs. That time may be significant when tests are running in parallel.

This PR modifies that test to explicitly measure the start of the test when the test function is entered and the end of the test when the local test run completes. This avoids accidentally measuring any initial wait time.

Author: grynspan

Tests/TestingTests/Traits/TimeLimitTraitTests.swift

@@ -152,15 +152,24 @@ struct TimeLimitTraitTests {
   @available(_clockAPI, *)
   @Test("Test does not block until end of time limit")
   func doesNotWaitUntilEndOfTimeLimit() async throws {
-    let timeAwaited = await Test.Clock().measure {
-      var configuration = Configuration()
-      configuration.testTimeLimitGranularity = .milliseconds(1)
-      configuration.maximumTestTimeLimit = .seconds(60)
+    var configuration = Configuration()
+    configuration.testTimeLimitGranularity = .milliseconds(1)
+    configuration.maximumTestTimeLimit = .seconds(60)
 
-      await Test {
-        try await Test.Clock.sleep(for: .nanoseconds(1))
-      }.run(configuration: configuration)
-    }
+    // Do not use Clock.measure {} here because it will include the time spent
+    // waiting for the test's task to be scheduled by the Swift runtime. We
+    // only want to measure the time from the start of the test until the call
+    // to run(configuration:) returns.
+    let timeStarted = Locked<Test.Clock.Instant?>()
+    await Test {
+      timeStarted.withLock { timeStarted in
+        timeStarted = .now
+      }
+      try await Test.Clock.sleep(for: .nanoseconds(1))
+    }.run(configuration: configuration)
+    let timeEnded = Test.Clock.Instant.now
+
+    let timeAwaited = try #require(timeStarted.rawValue).duration(to: timeEnded)
     #expect(timeAwaited < .seconds(1))
   }