Add tests for Safe-Linking

Add tests for Safe-Linking

Adding the test "tst-safe-linking" for testing that Safe-Linking works
as expected. The test checks these 3 main flows:
 * tcache protection
 * fastbin protection
 * malloc_consolidate() correctness

As there is a random chance of 1/16 that of the alignment will remain
correct, the test checks each flow up to 10 times, using different random
values for the pointer corruption. As a result, the chance for a false
failure of a given tested flow is 2**(-40), thus highly unlikely.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>