Investigate slow test root causes: OS vs solver

[tautschnig]

Add a focused CI workflow that runs the 4 tests marked THOROUGH in 6e616e9 across different OS × solver combinations to determine whether the slowness is inherent to the operating system or caused by the choice of SAT/SMT solver.

Test matrix:

• Tests 1-3 (StringSubstring, pid/C11, assigns-local-composite): {Linux, macOS-14} × {minisat2, cadical}
• Test 4 (large_array_of_struct_nondet_index): {Linux, Windows} × incremental SMT2

The 4 tests are temporarily changed back to CORE so CTest picks them up. All other PR-triggered workflows are disabled to minimize CI load.

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