selftests/clone3: fix number of tests in ksft_set_plan

Commit 515bddf0 ("selftests/clone3: test clone3 with CLONE_NEWTIME")
added an additional test, so the number passed to ksft_set_plan needs to
be bumped accordingly.

Also use ksft_finished() to print results and exit. This will catch future
mismatches between ksft_set_plan() and the number of tests being run.

Fixes: 515bddf0 ("selftests/clone3: test clone3 with CLONE_NEWTIME")
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>