btrfs-progs: tests: unify test drivers, make ready for extenral testsuite

btrfs-progs: tests: unify test drivers, make ready for extenral testsuite

Make the TOP variable more configurable, allow to set it to any path
where to find binaries when the testsuite is exported, or fallback to
system binaries.

There's now more code duplication, the logic is now more complex so it's
left open coded for clarity. Further cleanups are possible.

Signed-off-by: David Sterba <dsterba@suse.com>