Support multiple testcases for jdk_custom

[sophia-guo]

There has always be the requirement for supporting multiple testcases for jdk_custom target. Based on the solution for a different issue #3993 (comment) it might be worth to try and support that feature.

[sophia-guo]

multiple testcases can be separated by space.

CustomTarget in tap file doesn't support it yet. For example
TARGET=system_custom
CUSTOM_TARGET=-test=MathLoadTest -test-args="workload=math,timeLimit=5m"
https://ci.adoptopenjdk.net/view/Test_grinder/job/Grinder/6136/artifact/aqa-tests/TKG/output_16680060896447/Grinder_20221109145844.tap/*view*/

# timestamp: 20221109-150028
# repo dir: /home/jenkins/workspace/Grinder/aqa-tests/TKG/../system/aqa-systemtest
# git repo: 
#   Fetch URL: https://github.com/adoptium/aqa-systemtest.git
# sha:
# 767d954247d75a2df3736679f2c4e48130b825bb
# 
# Timestamp: Wed Nov  9 15:06:32 2022 UTC 
# CUSTOM_TARGET: -test=MathLoadTest                             CUSTOM_TARGET is cut off by space. 
1..1
ok 1 - system_custom_0 
  ---
    duration_ms: 302509

[sophia-guo]

Update documents :

• https://github.com/adoptium/aqa-tests/blob/master/buildenv/jenkins/testJobTemplate#L358