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

Symptom
-------

Timeouts running the test jdk/java/lang/String/concat/HiddenClassUnloading.java after JDK-8339166 on slower systems where 32m is the minimum heap size because 64k pages are configured.

Analysis
--------

JDK-8339166 increased the runtime of the test on a _fastdebug_ build significantly

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

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

Linux / ppc64le
After JDK-8339166: 12m30s
JDK-8339166 reverted: 2m30s

HiddenClassUnloading.java explicitly set -Xmx8m but the smallest heap possible on ppc64 is 32m with 64k pages:

./images/jdk/bin/java -Xlog:pagesize
[...]
[0.016s][info][pagesize] Heap: min=32M max=30208M base=0x00000000a0000000 size=30208M page_size=64K
[0.016s][info][pagesize] Block Offset Table: req_size=59M req_page_size=64K base=0x00007f2a398f0000 size=59M page_size=64K
[0.016s][info][pagesize] Card Table: req_size=59M req_page_size=64K base=0x00007f2a35df0000 size=59M page_size=64K
[0.016s][info][pagesize] Mark Bitmap: req_size=472M req_page_size=64K base=0x00007f2a185f0000 size=472M page_size=64K

On ppc64 with the larger heap no classes were onloaded before JDK-8339166. After JDK-8339166 the runtime with the fastdebug build increased even more than on other platforms.