test: add coverage for untested allocator behavior

[jvoisin]

Add small self-contained test cases and wire them into the Makefile and test_smc.py harness:

• invalid_free_aligned_sized_small: free_aligned_sized() with a non-power-of-two alignment, covering the "invalid sized deallocation alignment (small)" fatal path.
• invalid_malloc_object_size_small_canary: malloc_object_size() queried at an offset past the usable region, covering the "invalid malloc_object_size (canary)" fatal path.
• pvalloc: regression test for pvalloc(0) returning a valid allocation.
• reallocarray_overflow: reallocarray() returns NULL/ENOMEM on overflow.
• posix_memalign_einval: posix_memalign() returns EINVAL for an invalid alignment and leaves the output pointer untouched.
• aligned_alloc_einval: aligned_alloc() returns NULL/EINVAL for a non-power-of-two alignment.