RFR: 8311545: Allow test symbol files to be kept in the test image

[David Holmes]

Please review this simple change to allow a test to request that its symbol files be copied across into the test image.

Testing:
 - sanity test - tiers 1-3
 - direct testing in the context of [JDK-8311541](https://bugs.openjdk.org/browse/JDK-8311541)

Thanks

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Commit messages:
 - 8311545: Allow test symbol files to be kept in the test image

Changes: https://git.openjdk.org/jdk/pull/14797/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14797&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311545
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14797.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14797/head:pull/14797

PR: https://git.openjdk.org/jdk/pull/14797