tests generic mount

I want to skip tests on hosts where mount tries to deal with unsupported file systems.

Here I suggest a generic function ts_mount and provide example usage in cramfs and minix tests. If this is the way we want to go then I would consolidate the other tests too.

ts_mount needs to be very careful because it wants to interprete the output of the *untested* mount command.

For testing you could unload and blacklist cramfs or minix module.