btrfs-progs: tests: 001-simple-unmounted: fix test failure due to bashism

btrfs-progs: tests: 001-simple-unmounted: fix test failure due to bashism

The usage of 'source' is a bashism, and '.' should be used instead.  This
is causing fuzz-tests/001-simple-unmounted to fail in systems where
/bin/sh isn't bash:

    [TEST/fuzz]   001-simple-unmounted
./test.sh: 5: ./test.sh: source: not found
./test.sh: 7: ./test.sh: setup_root_helper: not found
./test.sh: 8: ./test.sh: check_prereq: not found
./test.sh: 18: ./test.sh: check_all_images: not found

Since most (all?) tests actually use /bin/bash, change this test to use
bash too.

Signed-off-by: Luis Henriques <henrix@camandro.org>
Signed-off-by: David Sterba <dsterba@suse.com>