Copied from the pull request:

Prior this change, most usages of the $(MKDIR) variable in jtreg's self-tests had to add -p in order to not fail with an "error if existing, make parent directories as needed". Many usages are lacking the flag and in concurrent test scenarios, this leads to flaky tests.

This PR appends -p to each expansion of $(MKDIR). Hard-coded -p will be removed; stray/duplicate -p flag don't seem to be a problem for mkdir's option parser. I didn't see any "-p specifed more than once" warnings.

This PR also cleans up redundant -f and -rf arguments from $(RM) ... calls, as they are very close in nature and location of the $(MKDIR) calls, as Defs.gmk declares RM to expand with -rf.

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Original issue description.

The output reads:

/bin/grep -s 'Test results: passed: 8' /home/runner/work/jtreg/jtreg/build/test/CleanupDirsTests.othervm.conc/jt.log > /dev/null
make: *** [../test/cleanupDirs/CleanupDirsTest.gmk:71: /home/runner/work/jtreg/jtreg/build/test/CleanupDirsTests.othervm.conc.ok] Error 1

Seen in:

- https://github.com/JornVernee/jtreg/actions/runs/14422233892/job/40446252292