We had several occasions where tests for the gitignore had been
added to status::ignore instead of the easier-to-handle
attr::ignore test suite. This most likely resulted from the fact
that the attr::ignore test suite is not easy to discover inside
of the attr folder. Furthermore, ignore being part of the
attributes code is an implementation detail, only, and thus
shouldn't be stressed as much.

Improve this by moving both attr::ignore and status::ignore tests
into a new ignore test suite.