test(3p/nix): Enable output comparison for evaluator success tests

Enables loading of the expected output of evaluator tests from the
corresponding .exp files, and checks that the output matches.

This again leaves some tests behind in the disabled folder, but we now
have almost the entire suite up and running so I can get around to
cleaning up the disabled ones.

Other note: Some tests had XML output, despite not being related to
XML testing at all - I'm not sure why they chose to do this, but have
converted those test outputs to normal Nix instead.

We have a separate test suite for JSON & XML serialisation already,
which was contributed by andi-.

Change-Id: Id7c42c836edfec4c22db9d893e35489f3e6dd559
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1285
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/default.nix

@@ -89,6 +89,7 @@ in pkgs.llvmPackages.libcxxStdenv.mkDerivation {
 
   installCheckPhase = ''
     export NIX_DATA_DIR=$out/share
+    export NIX_TEST_VAR=foo # this is required by a language test
     make test
   '';
 
@@ -124,6 +125,7 @@ in pkgs.llvmPackages.libcxxStdenv.mkDerivation {
 
   shellHook = ''
     export NIX_DATA_DIR="${toString depotPath}/third_party"
+    export NIX_TEST_VAR=foo
   '';
 
   # TODO(tazjin): integration test setup?