[olivhoenen]

Part of the split of #85 (Tests and Benchmarks for regression) into smaller, single-purpose PRs that are easier to review.

tests/testlowlevel.cpp and testlowlevel_c.c no longer compiled: they called al_begin_global_action without the datapath argument and used removed string tables. This fixes them, modernizes tests/CMakeLists.txt (link against al, register both programs with CTest using the ASCII backend) and adds an opt-in AL_BUILD_TESTS option (OFF by default). The C program also guards that al_lowlevel.h stays C-compatible. Bug fix only, no new tests.

Fork history: commit e7d8174 on the fork's develop.

Stack position: independent of the PRs above; base of the contract-test stack (the next PR builds on this branch).

Authorship: these commits are @yohannmarguier's, cherry-picked unchanged from his fork with git cherry-pick -x, so each one keeps its original author, date and message plus a (cherry picked from commit …) link back to the fork. Only conflicts with current develop and with the new commit order were resolved.

⚠️ Please merge with "Create a merge commit" or "Rebase and merge", not "Squash and merge": squashing would attribute Yohann's commits to the person who merges.

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