Test egginst

This PR adds a few tests for egginst, no behavior was changed. It does not test everything, but it should already cover a decent ground for some fragile parts of the code like entry points and 'proxy' support in egginst.

@rkern or @corranwebster could you take a look at this PR ? If you think the general approach makes sense, I will add more tests in the same spririt for both egginst and enstaller.