[jdk21u-dev] RFR: 8353953: com/sun/jdi tests should be fixed to not always require includevirtualthreads=y

[Rui Li]

A clean backport for JDK-8353953.

Backport for parity with oracle. `includevirtualthreads=y` is currently the default, but it's not desirable since it eliminates `includevirtualthreads=n testing coverage` and negates JDK-8282441. Remove the default and only provide `includevirtualthreads=y` to required tests. Tests are running. Low risk - test changes.

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Commit messages:
 - Backport d1d81dd01ca6f3fc1e4710e6055c5a3185f43d9a

Changes: https://git.openjdk.org/jdk21u-dev/pull/2335/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2335&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353953
  Stats: 17 lines in 4 files changed: 9 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2335.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2335/head:pull/2335

PR: https://git.openjdk.org/jdk21u-dev/pull/2335