Correct exit status of fish_run_tests.sh

fish_run_tests.sh was failing because its final command was a variable
test which was usually false. Switch to an if statement so the result is
true.

docker/context/fish_run_tests.sh

@@ -13,6 +13,11 @@ then
 fi
 
 ninja && ninja fish_run_tests
+RES=$?
 
 # Drop the user into a shell if FISH_RUN_SHELL_AFTER_TESTS is set.
-test -n "$FISH_RUN_SHELL_AFTER_TESTS" && exec bash -i
+if test -n "$FISH_RUN_SHELL_AFTER_TESTS"; then
+    bash -i
+fi
+
+exit $RES