Implement basic fuse mount tests.

Implement basic fuse mount tests.

The purpose of these tests is to verify that bcachefs fuse support works
as expected, including gathering valgrind errors from the bcachefs
executable.

To do this, bcachefs is executed from a helper thread in the new
util.BFuse class, which goes about setting up and validating the
valgrind data as well as making sure the mount has completed
sufficiently before the test starts executing.

This also includes some basic functionality smoke tests which cover file
creation, timestamps, etc.

Signed-off-by: Justin Husted <sigstop@gmail.com>