test: fs: Add filesystem integrity checks

test: fs: Add filesystem integrity checks

We need to make sure that file writes,file creation, etc. are properly
performed and do not corrupt the filesystem.
To help with this, introduce the assert_fs_integrity() function that
executes the appropriate fsck tool. It should be called at the end of any
test that modify the content/organization of the filesystem.
Currently only supports FATs and EXT4.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>