expand mirror test suite to cover all file/dir/fs ops

[chrisd8088]

Ensure all FUSE operations have a corresponding test in a test script (or program, in cases where a shell command is insufficient to test a particular operation). For example: chmod(), chown(), fsstat(), etc.

[kivikakk]

WIP on stat-tests branch for getattr.

[kivikakk]

I've done a few -- todo is still chown (awkward to test because we need to chown something on the lower filesystem as the test runner; do we assume sudo is usable and OK to use?), utimens, truncate, xattrs. Tests should run on both files and symlinks.

[chrisd8088]

Wowser! This is great, @kivikakk! I figured this might be somewhere down the road for us and I just didn't want to forget about it -- hence the issue -- but you've already made huge progress! ✨🙇

As for chown, what about something like a little shell function which did an id -nG, picked another group the user belonged to (assuming there is one), and then just did a chgrp on a file to that group? And maybe in the case where the user has only one group, the test could report a "skip" instead. I'm thinking of a format something like this [git test](https://github.com/git/git/blob/master/t/t1304-default-acl.sh:

if test ...; then
        test_set_prereq OTHER_GROUP
fi

test_expect_success OTHER_GROUP 'test chgrp' '...'

One could also just skip all remaining tests with something like:

if ! test_have_prereq OTHER_GROUP
then
    skip_all='skipping chown tests, no additional Unix group available'
    test_done
fi

Does that seem reasonable?

[kivikakk]

As for chown, what about something like a little shell function which did an id -nG, picked another group the user belonged to (assuming there is one), and then just did a chgrp on a file to that group?

I was thinking this would rarely fire because most users are only members of their own group, but then I forgot the state of Linux these days:

$ id -nG
kivikakk cdrom floppy sudo audio dip video plugdev netdev bluetooth scanner docker

This looks great, thank you!

[kivikakk]

Finally done in https://github.com/github/libprojfs/pull/13/commits/dde2c78c3bc768d6741cd07faeb945c02b303aea. God I hate shell scripting.