RFR: 7904154: JMH: Handle JVM flags from flags file

[Oli Gillespie]

When using a flags file, getInputArguments() doesn't include -XX: prefix which means the flags won't be valid in the forked VM. Find those flags and prefix them manually.

1. Added a test showing the issue, and covering a few different flag types.
2. Added logic to prefix flags with `-XX:` if necessary, allowing the test to pass.

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Commit messages:
 - Fix whitespace
 - 7904154: Handle JVM flags from flags file

Changes: https://git.openjdk.org/jmh/pull/168/files
  Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=168&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904154
  Stats: 157 lines in 2 files changed: 156 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jmh/pull/168.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/168/head:pull/168

PR: https://git.openjdk.org/jmh/pull/168