lib/tests/test-with-nix: run misc.nix tests in the derivation

This would have allowed us to catch to fromTOML regression in
[1] without waiting for the dogfooding on master, since previously
these tests [2] were not run for the Nix/Lix under test - only the host
nix.

[1]: https://github.com/NixOS/nix/pull/13741
[2]: https://github.com/NixOS/nixpkgs/pull/433710

(cherry picked from commit 67ef2657ff)

lib/tests/test-with-nix.nix

@@ -71,6 +71,9 @@ pkgs.runCommand "nixpkgs-lib-tests-nix-${nix.version}"
     echo "Running lib/tests/systems.nix"
     [[ $(nix-instantiate --eval --strict lib/tests/systems.nix | tee /dev/stderr) == '[ ]' ]];
 
+    echo "Running lib/tests/misc.nix"
+    [[ $(nix-instantiate --eval --strict lib/tests/misc.nix | tee /dev/stderr) == '[ ]' ]];
+
     mkdir $out
     echo success > $out/${nix.version}
   ''