Free-threading tests - #1136

Run the whole test suite with pytest-run-parallel.

• The new tests prove that the pattern

    spm.SentencePieceTrainer.train(...)
    sp = spm.SentencePieceProcessor()
    sp.Load(...)

is thread safe as long as multiple calls don't share the same objects; and that

• everything else - e.g. SentencePieceProcessor.Normalize() - is safe even while sharing the same SentencePieceProcessor object (this is because pytest-run-parallel does not parallelise setUp, so all threads use the same shared state).

Notable exclusion is the logstream= parameter, which is NOT thread safe. However it is undocumented and untested, which makes it fall out of scope of this PR.

Note how the stress testing performed by pytest-run-parallel is stochastic, and that on 4 threads it is unlikely to catch the more uncommon race conditions. However, I've extensively ran it on 32 threads locally and did not incur in any failures.