Tests: Don't cd to the tests directory!

We:

1. Set up a nice TMPDIR for our tests to use
2. Immediately `cd` to the directory containing the test runner.

So instead we don't do (2), and stay in the temp directory, and
explicitly use all the things from the test runner directory.

I am fairly certain that cmake papered over this by adding a second
layer of temp dir.

tests/checks/complete.fish

@@ -40,6 +40,8 @@ complete -c t -fa '(t)'
 complete -C't '
 # CHECK: t
 
+touch test.fish
+
 # Ensure file completion happens even though it was disabled above.
 complete -c t -l fileoption -rF
 # Only match one file because I don't want to touch this any time we add a test file.