test: Add tests for AsyncSemaphore and DispatchTimeInterval.

[Human-Directed AI Assistance]

Author: scottmarchant

Tests/DispatchAsyncTests/AsyncSemaphoreTests.swift

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+import Testing
+
+@testable import DispatchAsync
+
+@Test
+func asyncSemaphoreWaitSignal() async throws {
+    let semaphore = AsyncSemaphore(value: 1)
+
+    // First wait should succeed immediately and bring the count to 0
+    await semaphore.wait()
+
+    // Launch a task that tries to wait – it should be suspended until we signal
+    var didEnterCriticalSection = false
+    let waiter = Task {
+        await semaphore.wait()
+        didEnterCriticalSection = true
+        await semaphore.signal()
+    }
+
+    // Allow the task a brief moment to start and (hopefully) suspend
+    try? await Task.sleep(nanoseconds: 1_000)
+
+    #expect(!didEnterCriticalSection)  // should still be waiting
+
+    // Now release the semaphore – the waiter should proceed
+    await semaphore.signal()
+
+    // Give the waiter a chance to run
+    try? await Task.sleep(nanoseconds: 1_000)
+
+    #expect(didEnterCriticalSection)   // waiter must have run
+
+    _ = await waiter.value
+}

Tests/DispatchAsyncTests/DispatchTimeIntervalTests.swift

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+import Testing
+
+@testable import DispatchAsync
+
+@Test
+func dispatchTimeIntervalEquality() throws {
+    // 1 second == 1_000 milliseconds
+    #expect(DispatchTimeInterval.seconds(1) == .milliseconds(1_000))
+
+    // 1 second != 2 seconds
+    #expect(DispatchTimeInterval.seconds(1) != .seconds(2))
+
+    // 2_000 micro-seconds == 1 milliseconds
+    #expect(DispatchTimeInterval.microseconds(2_000) == .milliseconds(2))
+
+    // 1 micro-seconds == 1_000 nanoseconds
+    #expect(DispatchTimeInterval.microseconds(1) == .nanoseconds(1_000))
+
+    // `.never` is only equal to `.never`
+    #expect(DispatchTimeInterval.never == .never)
+    #expect(DispatchTimeInterval.never != .seconds(0))
+    #expect(DispatchTimeInterval.never != .seconds(Int.max))
+}