Add test for --ignore-dir=FILTER:FILTERARGS

[hoelzro]

Currently, we only test the --ignore-dir=DIRNAME form of the --ignore-dir option. Tests for the new syntax must be added. Since non-is filters don't work at the moment, these tests should be implemented but marked as TODO.

[andrewblack]

Which other filters might be supported in future. My guess is

• match - yes
• firstlinematch - no
• ext - no

Are there any other possible filters

[hoelzro]

@andrewblack Those are the only filters built into ack right now, so those are the only ones that need testing!

[hoelzro]

There are some new tests for this, but it could probably use some improvement.

[hoelzro]

One test that needs adding is making sure --ignore-dir=foo --ignore-dir=bar --noignore-dir=bar behaves correctly when foo/bar is encountered (it should search files under foo/bar)