test_major_sweep_does_not_create_oldmalloc_debt assumed the ring was
already old when the loop started. When Ruby is running with parse.y
this isn't always the case. That only held when GCs ran while
the ring was built. Under parse.y often none did, so the ring was
still young after GC.start.
This means that Minor GCs freed the buffers before they got promoted and
so there isn't any old garbage to sweep.
This commit runs GC.start RVALUE_OLD_AGE times before the loop, so every
buffer in the ring is old and each replacement leaves old garbage for
the next major to sweep, regardless of startup GC history.
Matt Valentine-House
e14d739e26