binary testing with llfuse/pyfuse3

[ThomasWaldmann]

$ ./borg.exe mount repo mnt
^ works

$ BORG_FUSE_IMPL=pyfuse3 ./borg.exe mount borg/repo/ borg/mnt2
borg mount not available: no FUSE support, BORG_FUSE_IMPL=pyfuse3.

Guess the binary was built with llfuse and only includes that.

When setting the env var, it tries to use pyfuse3, but it does not include that.

This is why the fuse3 tests fail with that binary.

[ThomasWaldmann]

fix after #9196.