selftests/exec: Add binfmt_script regression test

While working on commit b5372fe5 ("exec: load_script: Do not exec
truncated interpreter path"), I wrote a series of test scripts to verify
corner cases. However, soon after, commit 6eb3c3d0

 ("exec: increase
BINPRM_BUF_SIZE to 256") landed, resulting in the tests needing to be
refactored for the larger BINPRM_BUF_SIZE, which got lost on my TODO
list. During the recent exec refactoring work[1], the need for these tests
resurfaced, so I've finished them up for addition to the kernel selftests.

[1] https://lore.kernel.org/lkml/202005191144.E3112135@keescook/

Link: https://lkml.kernel.org/r/202005200204.D07DF079@keescook
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>