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-regex-split.exp

@@ -1 +0,0 @@
-true