capsicum-test: improve & unify test instructions

Remove one-off scripts.

Transfer content from tools/testing/capsicum_tests/README to
main capsicum-linux README.md.

Document more details on building a suitable UML disk image.
capsicum: always return ERR_PTR, not NULL

The fget_raw_no_unwrap() arm for a bad file descriptor
was still returning NULL, but all callers have been
converted to expect an ERR_PTR.