Add methods for path lookups in test_install_{extras, index}.py

[ssurbhi560]

towards #6050

[gutsytechster]

You've not provided the message arguments at some places. That's the only thing I guess, which is left, else LGTM :) Please correct if I get some reviews wrong.
Please find the reviews inline.

[ssurbhi560]

@deveshks @gutsytechster , Thank you for the review, will make these changes!

[pradyunsg]

I don't think any of the suggested changes are appropriate. All of them are suggesting setting message to something that's already included in str(result) in a much more readable form.

message should be a human readable string describing the error. To reinforce that idea, I strongly suggest only passing it as a keyword argument (the only reason I didn't make it one in the original PR was Python 2 support 😞).

message=result.files_created doesn't read right, does it? :P

Instances like these, where not-all-the-relevant-info messages were provided through the assertion are spread across the test suite (str(result) includes all the context, result.files_created doesn't), and fixing them to provide all the relevant information is the point of this exercise (in addition to the fact that we'd have 1 spot to normalize how paths vs strings are handled in such comparisions).

[ssurbhi560]

I was thinking somewhat the same, but wasn't confident.
Thank you @pradyunsg for reviewing, will update the PR. :)

[gutsytechster]

Eh! Thanks for correcting. I'll update my PR as well. :P