tests: test the tests

Adds a regression test for #2076. This test ensures that
`extraConfigLua` is used in `finalPackage` and that the test will fail
correctly when running `nvim` results in unexpected output.

flake-modules/tests.nix

@@ -25,6 +25,11 @@
           inherit (self.lib.${system}.check) mkTestDerivationFromNixvimModule;
         };
 
+        failing-tests = import ../tests/failing-tests.nix {
+          inherit pkgs;
+          inherit (self.lib.${system}.check) mkTestDerivationFromNixvimModule;
+        };
+
         no-flake = import ../tests/no-flake.nix {
           inherit system;
           inherit (self.lib.${system}.check) mkTestDerivationFromNvim;

tests/failing-tests.nix

@@ -0,0 +1,22 @@
+{
+  pkgs,
+  mkTestDerivationFromNixvimModule,
+}:
+let
+  inherit (pkgs.testers) testBuildFailure;
+
+  failed = testBuildFailure (mkTestDerivationFromNixvimModule {
+    name = "prints-hello-world";
+    module = {
+      extraConfigLua = ''
+        print('Hello, world!')
+      '';
+    };
+    inherit pkgs;
+  });
+in
+pkgs.runCommand "failing-test" { inherit failed; } ''
+  grep -F 'Hello, world!' "$failed/testBuildFailure.log"
+  [[ 1 = $(cat "$failed/testBuildFailure.exit") ]]
+  touch $out
+''