Exclude tst-realloc from tests-mcheck

The realloc (NULL, 0) test in tst-realloc fails with gcc 7.x but
passes with newer gcc.  This is because a newer gcc transforms the
realloc call to malloc (0), thus masking the bug in mcheck.

Disable the test with mcheck for now.  The malloc removal patchset
will fix this and then remove this test from the exclusion list.

Reported-by: Stefan Liebler <stli@linux.ibm.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>