Delete valgrind magic in pss-test.

Delete valgrind magic in pss-test.

* testsuite/pss-test.c: Delete magic to let valgrind to check if
pss_encode_mgf1 is side-channel silent with respect to the salt
and digest inputs. It turns out that the most significant bits of
the padded bignum, and hence its size, depends on these inputs.
Which results in a data-dependent branch in the normalization code
of at the end of gmp's mpz_import.