Port config tests to littlecheck

This required a bit of thinking.

What we do is we have one test that fakes $HOME, and then we do the
various config tests there.

The fake config we have is reused and we exercise all of the same codepaths.

tests/checks/broken-config/fish/config.fish

@@ -0,0 +1,4 @@
+echo broken
+if status is-login
+    syntax-error
+end