RFR: 8324966: Allow selecting jtreg test case by ID

[Axel Boldt-Christmas]

[JDK-8287828](https://bugs.openjdk.org/browse/JDK-8287828) added support for selecting specific JTREG tests cases by their ID. However because of how we handle input strings in make it was not possible to use `#` anywhere, breaking this feature.

Prior to this change
 * `TEST="gc/TestSystemGC.java#Serial gc/TestSystemGC.java#G1" make test` Works.
 * `make test TEST="gc/TestSystemGC.java#Serial gc/TestSystemGC.java#G1"` Does not work.

After this change both works.

When propagating command line variables through the make system we transiently replaced spaces with `#` (which drops any actual `#` when restoring the spaces). This patch replaced the `#` character with `§` under the assumption that it will not be used in these arguments.

This works for now. An alternative would be to make this more robust by selecting a sequence of characters that is checked to not be part of the strings in question as the space placeholder. But I will leave that to our more advance Make engineers to handle.

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Commit messages:
 - 8324966: Allow selecting jtreg test case by ID

Changes: https://git.openjdk.org/jdk/pull/19963/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19963&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324966
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/19963.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19963/head:pull/19963

PR: https://git.openjdk.org/jdk/pull/19963