RFR: 8347817: timeouts running test/jdk/java/lang/String/concat/HiddenClassUnloading.java with fastdebug builds

[Richard Reingruber]

This PR reverts the fix from [JDK-8339166](https://bugs.openjdk.org/browse/JDK-8339166) because it increases the runtime of the test a lot.
Instead a full gc is requested via the whitebox api. This solves the issues (see bug description) and it is also clearer and more reliable that allocating objects and classes expecting a gc because of the small heap.
The heap setting is also removed since it is not needed anymore.

#### Testing: Duration Running HiddenClassUnloading with fastdebug Builds

I'm giving the minimum of a few runs because durations vary by +/- 10s.


Linux / x86_64:
After JDK-8339166:    3m15s
JDK-8339166 reverted:   50s
PR:                     50s

macOS / M1:
After JDK-8339166:    2m30s
JDK-8339166 reverted:   30s
PR:                     30s

Linux / ppc64le
After JDK-8339166:    12m30s
JDK-8339166 reverted:  2m30s  (failure: no classes unloaded)
PR:                    2m30s  (success)

-------------

Commit messages:
 - Call WhiteBox::fullGC() for class unloading
 - Revert "8339166: java/lang/String/concat/HiddenClassUnloading.java fails on AIX and Linux ppc64le after JDK-8336856"

Changes: https://git.openjdk.org/jdk/pull/23137/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23137&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347817
  Stats: 13 lines in 1 file changed: 10 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23137.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23137/head:pull/23137

PR: https://git.openjdk.org/jdk/pull/23137