RFR: 8331939: Add custom hook for TestImage

[Erik Joelsson]

We need a custom hook in TestImage.gmk to modify behavior when building with custom source.

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Commit messages:
 - JDK-8331939

Changes: https://git.openjdk.org/jdk/pull/19139/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19139&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331939
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19139.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19139/head:pull/19139

PR: https://git.openjdk.org/jdk/pull/19139