crypto: Advanced test case for spkac functions

Moved away from using file based PEM & SPKAC for tests to object based
comparisons of example SPKAC's using key sizes ranging from 1024 - 4096,
and hashing algorithms md4, md5, sha1, sha224, sha256, sha384, sha512 &
rmd160

crypto: Old fixture cleanup

Removed old fixtures used in test case.
test/fixtures/spkac.pem
test/fixtures/spkac.valid
test/fixtures/spkac.fail

Pull request #6367 implements new test case