DockerTestUtils::execute shows incorrect elapsed time

DockerTestUtils::execute calculates the elapsed time without waiting for the child process to finish execution:

https://github.com/openjdk/jdk/blob/704b9a66bba0dc8adb62be80fd62864b9c687c3f/test/lib/jdk/test/lib/containers/docker/DockerTestUtils.java#L282

The fix is to call output.getStdout() first, which waits for the child process to finish execution.