RFR: 8287352: DockerTestUtils::execute shows incorrect elapsed time

[Ioi Lam]

Please review this small change to improve the test reporting:

- Calculating elapsed time after child process has exited
- Do not print `"Child process STDOUT is trimmed ..."` unless the lines are really trimmed.

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Commit messages:
 - 8287352: DockerTestUtils::execute shows incorrect elapsed time

Changes: https://git.openjdk.java.net/jdk/pull/8897/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8897&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287352
  Stats: 11 lines in 1 file changed: 5 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8897.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8897/head:pull/8897

PR: https://git.openjdk.java.net/jdk/pull/8897