KEEP_REPORTDIR does not work as expected

When KEEP_REPORTDIR is set to false, we should not capture any passed test artifacts. However, this does not work as expected. Please see the build link below:
https://ci.eclipse.org/openj9/job/Test_openjdk8_j9_extended.functional_s390x_linux_aot_Personal_testList_0/4/

It looks like all test artifacts are captured.

I think this is due to test pipeline uses boolean https://github.com/AdoptOpenJDK/openjdk-tests/blob/e55242930ee3bd56ceb2cbdcb898df130fbfc9c9/buildenv/jenkins/JenkinsfileBase#L78
https://github.com/AdoptOpenJDK/openjdk-tests/blob/master/buildenv/jenkins/JenkinsfileBase#L264

and TKG uses string https://github.com/AdoptOpenJDK/TKG/blob/b7d19e42ab2c131d71ba037304b6efd8e07d5ad5/settings.mk#L195

Start with the KEEP_REPORTDIR handling at buildenv/jenkins/JenkinsfileBase lines 78 and 264, then compare it with TKG's settings.mk line 195. Use the linked CI build as the failing example and verify that setting KEEP_REPORTDIR to false does not capture passed-test artifacts while the enabled behavior remains unchanged.

Written by the indexing model from the issue text.