test(tvix/eval): eval-okay-eq.nix can actually be tested

eval-okay-eq.nix is actually an ancient test case that used the ATerm
syntax for the test result. It was disabled for a while, since the
behavior got reverted for a bit, but works on any reasonably modern
Nix implementation. This change matches my [C++ Nix PR].

[C++ Nix PR]: https://github.com/NixOS/nix/pull/7196

Change-Id: I602fd7c83a0bc104ab502c8b6a74e4591272be1a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7045
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/nix_tests/eval-okay-eq.exp

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

tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq.exp

@@ -1 +0,0 @@
-Bool(True)