RFR: CODETOOLS-7903568: Allow `-show:stream file.jtr`

[Jonathan Gibbons]

Please review an update to permit a new command-line form

     jtreg -show:stream path/to/test.jtr

Currently, the desired test can only be specified via the "standard" options for a test suite and path within a test suite.
This change makes it possible to specify a single `.jtr` file directly. This may be particularly useful to extract the `rerun` output that is within each `.jtr` fie.

-------------

Commit messages:
 - CODETOOLS-7903568: Allow `-show:stream file.jtr`

Changes: https://git.openjdk.org/jtreg/pull/176/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=176&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903568
  Stats: 133 lines in 5 files changed: 97 ins; 30 del; 6 mod
  Patch: https://git.openjdk.org/jtreg/pull/176.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/176/head:pull/176

PR: https://git.openjdk.org/jtreg/pull/176