Cleanup test setup a bit

Put the env setup into test_util, which does some additional work.

Also use some more builtins and stuff.

tests/test.fish

@@ -19,22 +19,7 @@ else
     set files_to_test checks/*.fish
 end
 
-# These env vars should not be inherited from the user environment because they can affect the
-# behavior of the tests. So either remove them or set them to a known value.
-# See also tests/interactive.fish.
-set -gx TERM xterm
-set -e COLORTERM
-set -e INSIDE_EMACS
-set -e ITERM_PROFILE
-set -e KONSOLE_PROFILE_NAME
-set -e KONSOLE_VERSION
-set -e PANTHEON_TERMINAL_ID
-set -e TERM_PROGRAM
-set -e TERM_PROGRAM_VERSION
-set -e VTE_VERSION
-set -e WT_PROFILE_ID
-set -e XTERM_VERSION
-
+# test_util handles the environment setup and then restarts us
 source test_util.fish (status -f) $argv
 or exit