[test] test OpenSshConfigFile directly, not via the JSch config

[test] test OpenSshConfigFile directly, not via the JSch config

This is a prerequisite for removing the JSch support bundle; otherwise
OpenSshConfigFile would be left without tests.

Copy OpenSshConfigTest from the JSch support bundle and adapt all tests
to perform the equivalent checks on OpenSshConfigFile directly. Add a
new lookupDefault() method to the SshConfigStore interface and implement
it so that it behaves the same and the tests work identically.

Change-Id: I046abd9197a8484003e77005024e5d973456f1a3