Doing cleanup in central_freelist_test. Changing num_spans to kNumSpans and its type from const to constexpr.
Doing cleanup in central_freelist_test. Changing num_spans to kNumSpans and its
type from const to constexpr.

PiperOrigin-RevId: 431553795
Change-Id: Id3943e74065a32ea019e6057e3966c04e8d38260

google/tcmalloc,google/tcmalloc,google/tcmalloc