StopWatchTests: improve test cases

Uncomment flaky assertions and replace with deterministic implementation

Signed-off-by: Vincent Stradiot <vincentstradiot@hotmail.com>

spring-core/src/test/java/org/springframework/util/StopWatchTests.java

@@ -23,7 +23,9 @@ import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 import org.springframework.util.StopWatch.TaskInfo;
 
-import static org.assertj.core.api.Assertions.*;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.Mockito.when;
 
 /**