RFR: JDK-8274170: Add hooks for custom makefiles to augment jtreg test execution

[Mikhailo Seledtsov]

Please review this small change that adds hook for custom makefiles to augment parameters for jtreg test execution.

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Commit messages:
 - Initial change

Changes: https://git.openjdk.java.net/jdk/pull/5666/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5666&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274170
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5666.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5666/head:pull/5666

PR: https://git.openjdk.java.net/jdk/pull/5666