selftests: fix some typos in tools/testing/selftests

Fix multiple spelling errors:

 - "rougly" -> "roughly"
 - "fielesystems" -> "filesystems"
 - "Can'" -> "Can't"

Link: https://lkml.kernel.org/r/20250503211959.507815-1-chelsyratnawat2001@gmail.com


Signed-off-by: Chelsy Ratnawat <chelsyratnawat2001@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>