crypto/* tests: Fix some gcc -Wdiscarded-qualifiers warnings. * tests/test-gc-hmac-md5.c (main): Change type of variables with a string literal initializer to 'const char *'. * tests/test-gc-hmac-sha1.c (main): Likewise. * tests/test-gc-hmac-sha256.c (main): Likewise. * tests/test-gc-hmac-sha512.c (main): Likewise. * tests/test-gc-md2.c (main): Likewise. * tests/test-gc-md5.c (main): Likewise. * tests/test-gc-sha1.c (main): Likewise. * tests/test-gc-sha256.c (main): Likewise. * tests/test-gc-sha512.c (main): Likewise. * tests/test-hmac-md5.c (main): Likewise. * tests/test-hmac-sha1.c (main): Likewise. * tests/test-hmac-sha256.c (main): Likewise. * tests/test-hmac-sha512.c (main): Likewise.