RFR: 7904066: Make some plugin tests runnable with JTReg

[Alexandre Iline]

Adding necessary TEST.ROOT files.
Fixing up a few tests.
Fixing a problem in the code.

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Commit messages:
 - CODETOOLS-7904066: Make some plugin tests runnable with JTReg

Changes: https://git.openjdk.org/jcov/pull/64/files
  Webrev: https://webrevs.openjdk.org/?repo=jcov&pr=64&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904066
  Stats: 100 lines in 7 files changed: 78 ins; 6 del; 16 mod
  Patch: https://git.openjdk.org/jcov/pull/64.diff
  Fetch: git fetch https://git.openjdk.org/jcov.git pull/64/head:pull/64

PR: https://git.openjdk.org/jcov/pull/64