tests/misc-fs-stress: Provide it with a default file path

Currently, a file path always need to be specified as an argument,
so let's make things easier by providing a default file path.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>