Fix selftest broken by 5c272847d0d (it wanted to use flip as a var)

More tweaks to testutils

- change API of check_HOL_ERR to be tupled rather than curried (makes
it easier to write the (fn _ => true) predicate)
- add check_result entrypoint

Tweak testutils

- output on failures now prettier
- more utility functions
- make 'a testresult same as 'a Exn.result

Fix meson selftest

It was assuming in more places than I first thought that numbers were
in scope, and I'd only tested it against hol rather than hol.bare.

Create a selftest version of old src/meson/test.sml

Document behaviour in terms of "success" or "expected to fail" based
only on what happens with HOL as of this commit.