assert: fix TestEventuallyTimeout

Fix TestEventuallyIssue805 which was flaky because of the use of a timer
that has exactly the same duration as the Eventually timout. But time is
not a synchronization primitive.

Now we use channels to ensure that the condition is still running and
Eventually times out before checking its return value.

The test is also renamed to TestEventuallyTimeout to more clearly show
its purpose.

assert/assertions_test.go

@@ -2870,12 +2870,31 @@ func TestNeverTrue(t *testing.T) {
 	False(t, Never(mockT, condition, 100*time.Millisecond, 20*time.Millisecond))
 }
 
-func TestEventuallyIssue805(t *testing.T) {
+// Check that a long running condition doesn't block Eventually.
+// See issue 805 (and its long tail of following issues)
+func TestEventuallyTimeout(t *testing.T) {
 	mockT := new(testing.T)
 
 	NotPanics(t, func() {
-		condition := func() bool { <-time.After(time.Millisecond); return true }
+		t.Log("start")
+		done, done2 := make(chan struct{}), make(chan struct{})
+
+		// A condition function that returns after the Eventually timeout
+		condition := func() bool {
+			t.Log("ok1")
+			// Wait until Eventually times out and terminates
+			<-done
+			close(done2)
+			t.Log("ok2")
+			return true
+		}
+
 		False(t, Eventually(mockT, condition, time.Millisecond, time.Microsecond))
+
+		t.Log("Eventually done")
+		close(done)
+		<-done2
+		t.Log("Test done")
 	})
 }