tests: better testing of loaded certificates
Tests were failing on systems like RHEL 7 where loading the system
certificates results in CA certs being reported to Python. We add
a feature that detects when we're able to load *and detect* the
loading of system certificates. We update the tests to cover the
3 scenarios:
1) system CAs are loadable and detected
2) system CAs are loadable but not detected
3) system CAs aren't loadable
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown