test_fs: Allow running unprivileged

test_fs: Allow running unprivileged

There is no need to mount the filesystem on the host side.
All filesystem tools offer some way to fill the fs without mounting.

So, create the content on the host side, create and fill the fs
without mounting.
No more sudo or guestmount needed.

This new approach works because the tests don't care about user IDs
and no device files are needed.
If user IDs start to matter it's still possible to use wrapper
tools like fakeroot in future while filling the fs.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>