Adds an xtmtest-with-fixture macro that sets up a fixture and then
runs its body in the fixture's environment.

There's a bunch of stuff introducing conveniently named macros within
the body, which got added as I was debugging and getting the basic macro
to work and it's a tad tricky to unpick this and present it as a series of
patches adding small pieces of behaviour every time. Sorry.

I thought, I'll fix the double eval in xtmtest and xtmtest-compile by
checking if the `form` argument is quoted, stripping the quote, and then
doing `(eval ',form (interaction-environment))` in the body of the test.

By my understanding of the way lisps work, this should not change behaviour in
any way.

Hah! Tests are now failing. What on _earth_ am I doing wrong?

squash! Fail a sanity check

We're still doing a double eval, but that's only until we fix the tests.

Since we've removed the double eval from xtmtest and xtmtest-compile,
we can now fix all the tests to stop quoting their 'form' arguments.

'tests/all-core.xtm' is still passing.

Since we've fixed the tests so they don't quote the 'form' argument,
we can remove the belt and braces code that unquoted such forms before
expanding the test macros as well.