Both tests operate on %T/testbin. If the two tests run concurrently,
one may fail.
This is likely the root cause of flaky failures reported by
https://lists.llvm.org/pipermail/llvm-dev/2020-September/144781.html
https://llvm.org/docs/CommandGuide/lit.html says:
%T parent directory of %t (not unique, deprecated, do not use)