RFR: 8279022: JCmdTestFileSafety.java should check file time stamp for test result

[Yumin Qi]

When test finished, the created archive file should be checked after  the first run for existence, and checked for time stamp after second run.  Since the file name is same, this method is more accurate than the existing checking, only checking the existence of the file. Since jcmd can do repeat dynamic dump to same java process now, removed unnecessary restart process code.

Tests: tier1, jtreg on local linux-x64.

Thanks
Yumin

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Commit messages:
 - Revise output message
 - 8279022: JCmdTestFileSafety.java should check file time stamp for test result

Changes: https://git.openjdk.java.net/jdk/pull/6904/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6904&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279022
  Stats: 29 lines in 1 file changed: 18 ins; 2 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6904.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6904/head:pull/6904

PR: https://git.openjdk.java.net/jdk/pull/6904