Skip MD5 certificate tests.

Beyond obsolete and now rejected by BoringSSL, e.g. if a current
version is present via mainline.

This was fixed properly in change If2a4739cd7b89e65136e5e2b50d1af810930ee6f
but that doesn't backport cleanly so just skipping these tests on older
CTS as MD5 certs have been rejected for Android TLS for many years now
anyway.

Bug: 286993265
Bug: 286813963
Test: atest CtsLibcoreTestCases:tests.targets.security.cert.CertificateTest
Change-Id: If2a4739cd7b89e65136e5e2b50d1af810930ee6f
Merged-In: If2a4739cd7b89e65136e5e2b50d1af810930ee6f