malloc: Fix -Wuse-after-free warning in tst-mallocalign1 [BZ #26779]

The test leaks bits from the freed pointer via the return value
in ret, and the compiler correctly identifies this issue.
We switch the test to use TEST_VERIFY and terminate the test
if any of the pointers return an unexpected alignment.

This fixes another -Wuse-after-free error when compiling glibc
with gcc 12.

Tested on x86_64 and i686 without regression.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>