tests, add function ts_mount

tests, add function ts_mount

We want to generalize mounts similar to how we do it already in
minix/mkfs. Currently most of our other checks just fail if
"fs not supported".

Note that the new function checks a bit stronger than before in
minix/mkfs. It respects mount's return value and stderr goes
to $TS_OUTPUT too.

It's a bit ugly that we write to $TS_OUTPUT from inside of the
function. But if the caller would do it then we couldn't
ts_skip and the whole function would be pointless.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>