fix(depotfmt): only exclude *imported* tvix tests

When we added the Nix language test suite in cl/6126, we excluded the
whole tvix tests folder from Nix formatting. This is unintentional, as
we probably want *our* tests to be formatted correctly.

Change-Id: I2b66d79e30fae17e75d5a1f8c44e279886091c5f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11154
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-fail-remove.nix

@@ -1,5 +1,5 @@
 let {
-  attrs = {x = 123; y = 456;};
+attrs = { x = 123; y = 456; };
 
-  body = (removeAttrs attrs ["x"]).x;
-}
+body = (removeAttrs attrs [ "x" ]).x;
+}