Manager: add canary test to timeout checker unit test

The `TestTaskTimeout()` unit test assumes specific durations for initial &
subsequent sleeps of the timeout checker. The test will fail quite
cryptically when that assumption doesn't hold, so just test for it at
the start of the unit test.

internal/manager/timeout_checker/timeout_checker_test.go

@@ -82,6 +82,13 @@ func TestTimeoutCheckerTiming(t *testing.T) {
 }
 
 func TestTaskTimeout(t *testing.T) {
+	// Canary test: if these constants do not have the expected value, the test
+	// will fail rather cryptically.
+	if !assert.Equal(t, 5*time.Minute, timeoutInitialSleep, "timeoutInitialSleep does not have the expected value") ||
+		!assert.Equal(t, 1*time.Minute, timeoutCheckInterval, "timeoutCheckInterval does not have the expected value") {
+		t.FailNow()
+	}
+
 	ttc, finish, mocks := timeoutCheckerTestFixtures(t)
 	defer finish()