std.fs needs more tests

[squeek502]

From #5538:

Would be nice to add test coverage for this

Agreed. All of std.fs (and other fs related stuff) is rather undertested imo; might take a stab at improving coverage overall at some point.

Currently, std.fs only has a few tests here; a lot is still completely untested. This is leading to regressions like #5643 and #5648 (@andrewrk ran the tests before merging b9e3df9 but none of the tests actually cover any of the affected code, so regressions slipped through)

[squeek502]

I think this can be closed. There's room for improvement still, but it's progressed considerably over where it was when this issue was created.

For reference, here's what the coverage of fs.zig currently looks like on Linux for the std lib tests (from the end of this blog post):

File                                 Covered LOC Executable LOC Coverage
------------------------------------ ----------- -------------- --------
lib/std/fs.zig                       351         449              78.17%

And here's the line-by-line coverage:

https://gist.github.com/squeek502/b5479e37f395509cb26d6bdc450d13d8

(note that unused functions are interpreted as non-executable, so there are (mostly Absolute-variety) functions that remain totally untested that don't show up as uncovered in these results)