Make tests usable with path with spaces

This is somewhat subtle:

The #RUN line in a littlecheck file will be run by a posix shell,
which means the substitutions will also be mangled by it.

Now, we *have* shell-quoted them, but unfortunately what we need is to
quote them for inside a pre-existing layer of quotes, e.g.

    # RUN: fish -C 'set -g fish %fish'

here, %fish can't be replaced with `'path with spaces/fish'`, because
that ends up as

    # RUN: fish -C 'set -g fish 'path with spaces/fish''

which is just broken.

So instead, we pass it as a variable to that fish:

    # RUN: fish=%fish fish...

In addition, we need to not mangle the arguments in our test_driver.

For that, because we insist on posix shell, which has only one array,
and we source a file, we *need* to stop having that file use
arguments.

Which is okay - test_env.sh could previously be used to start a test,
and now it no longer can because that is test_*driver*.sh's job.

For the interactive tests, it's slightly different:

pexpect.spawn(foo) is sensitive to shell metacharacters like space.

So we shell-quote it.

But if you pass any args to pexpect.spawn, it no longer uses a shell,
and so we cannot shell-quote it.

There could be a better way to fix this?

tests/checks/rc-returned.fish

@@ -1,2 +1,2 @@
-#RUN: %fish -c '%fish -c false; echo RC: $status'
+#RUN: fish=%fish %fish -c '$fish -c false; echo RC: $status'
 # CHECK: RC: 1