CI: run tests in parallel for all targets

[WojciechMula]

Is there anything that prevents us from running ctest -j for the following targets:

$ simdutf/.github/workflows$ grep ctest * | grep -v j
alpine.yml:          ./alpine.sh bash -c "cd build_for_alpine && ctest"
armv7.yml:            ctest --output-on-failure --test-dir build
macos-latest.yml:          ctest --output-on-failure
s390x.yml:            ctest --output-on-failure --test-dir build
ubuntu22sani.yml:          ctest --output-on-failure
ubuntu22sani.yml:          ctest --output-on-failure
vs16-ci.yml:        ctest -C Release   --output-on-failure
vs16-ci.yml:        ctest -C Debug  --output-on-failure -E basic_fuzzer
vs17-ci-cxx20.yml:        ctest -C Release   --output-on-failure
vs17-ci-cxx20.yml:        ctest -C Debug  --output-on-failure  -E basic_fuzzer
vs17-ci.yml:        ctest -C Release   --output-on-failure
vs17-ci.yml:        ctest -C Debug  --output-on-failure  -E basic_fuzzer
vs17-clang-ci.yml:        ctest -C Release   --output-on-failure
vs17-clang-ci.yml:        ctest -C Debug  --output-on-failure  -E basic_fuzzer

[lemire]

Parallel runs are more likely to run out of memory and fail for spurious reasons (ressouces exhaution). But my instinct in these cases is that we could parallelize without harm.

[WojciechMula]

Yes, there might be some problems if we set # of processes too large. But we always can setup some decent limit on parallelization.

I propose to enable single target per PR, and we'll learn which targets can safely benefit for the -j switch.