Add self-contained ASN.1 parsing tests, so that ASN.1 parsing is not
solely tested through X.509 and TLS.
The tests cover every function and almost complete line coverage in
asn1parse.c.
A few test cases containing negative and edge case INTEGER values are
deliberately deactivated because the historical library behavior is at
odds with official specifications, but changing the behavior might
break interoperability.
Other than that, these tests revealed a couple of minor bugs which
will be fixed in subsequent commits.
