Add common mount options and fix most of the tests for fuse-t

[macos-fuse-t]

go test ./... still freezes because of concurrency issues in the testing infra. However most of the tests are passing when running one by one.

[anacrolix]

I made it store the backend per Conn, and fixed some variable scoping.

The race is caused by checking the ready chan after mounting for a MountError. The FUSE-T code isn't synchronizing the same way the OSXFUSE stuff does. I simple attempt to fix it didn't work for me, it needs a deeper look.

AFAICT, there might be some issues with macfuse 4.5 not handling some mount flags, even if I apply the minimal fixes since I forked from zegl, those issues still come up. This is a problem with the OSXFUSE implementation however, and probably not a concern for FUSE-T.

[anacrolix]

I am using the tests in github.com/anacrolix/torrent/fs as the ultimate screen for whether the different fuse backends are working. Currently both backends (OSXFUSE/MacFUSE, and FUSE-T) work on Mac.