RFR: 8198317: Enhance JavacTool.getTask for flexibility

[Jan Lahoda]

This is a clone of https://github.com/openjdk/jdk/pull/1896, which got closed by Skara.

@lgxbslgx, please let me know if you would prefer to resubmit the PR yourself.

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Commit messages:
 - Revise test code. Fix typo.
 - Revise test.
 - Add a test case.
 - 8198317: Enhance JavacTool.getTask for flexibility

Changes: https://git.openjdk.java.net/jdk/pull/2871/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2871&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8198317
  Stats: 137 lines in 2 files changed: 135 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2871.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2871/head:pull/2871

PR: https://git.openjdk.java.net/jdk/pull/2871