faster 32-bit CI tests

[Cyan4973]

this is the longest CI test, reaching ~40mn on last PR, so we are waiting CI completion on this one currently.

[Cyan4973]

It reduced the target test to 20mn.
Still long, but better.

Our new longest test is now clang-msan-testzstd (29mn)

[Cyan4973]

make-test-32bit is now down to 12 minutes, no longer an issue.

Good, let's now have a look at the next longest one, clang-msan-testzstd

edit: this test is run entirely within playTests.sh, which is a single sh script, serially executed by nature.
Parallel execution would require breaking this test into multiple smaller parts, that could be started in parallel.
This will have to be a separate PR.