feat(tvix/eval): expect not-yet-passing tests to fail

It is helpful to be able to use the test suite as a regression test:
make a change to the compiler/vm, re-run the tests, and if there are
any failures you know it's your fault.

Right now we can't do that, because the expected-to-fail tests are
mixed in with the expected-to-pass tests.  So we can't use them as a
regression test.

Change-Id: Ied606882b9835a7effd7e75bfcf3e5f827e0a2c8
Signed-off-by: Adam Joseph <adam@westernsemico.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7036
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/tests/nix_tests/eval-okay-closure.nix

@@ -1,13 +0,0 @@
-let
-
-  closure = builtins.genericClosure {
-    startSet = [{key = 80;}];
-    operator = {key, foo ? false}:
-      if builtins.lessThan key 0
-      then []
-      else [{key = builtins.sub key 9;} {key = builtins.sub key 13; foo = true;}];
-  };
-
-  sort = (import ./lib.nix).sortBy (a: b: builtins.lessThan a.key b.key);
-
-in sort closure