tst-realloc: do not check for errno on success [BZ #22611]

POSIX explicitly says that applications should check errno only after
failure, so the errno value can be clobbered on success as long as it
is not set to zero.

Changelog:
	[BZ #22611]
	* malloc/tst-realloc.c (do_test): Remove the test checking that errno
	is unchanged on success.