tests.buildenv: init test harness

Add an eval-time test harness for buildEnv following the pattern established by `tests.overriding`.
Uses `lib.runTests` to compare expr/expected pairs, and the buildCommand reports pass/fail.

No tests yet; subsequent commits add them individually.

pkgs/test/buildenv.nix

@@ -0,0 +1,52 @@
+{
+  lib,
+  pkgs,
+  stdenvNoCC,
+}:
+
+let
+  inherit (pkgs) buildEnv;
+
+  testingThrow = expr: {
+    expr = (builtins.tryEval (builtins.seq expr "didn't throw"));
+    expected = {
+      success = false;
+      value = false;
+    };
+  };
+
+  tests = { };
+in
+
+stdenvNoCC.mkDerivation (finalAttrs: {
+  __structuredAttrs = true;
+  name = "test-buildenv";
+  passthru = {
+    inherit tests;
+    failures = lib.runTests finalAttrs.passthru.tests;
+  };
+  testResults = lib.mapAttrs (_: test: test.expr == test.expected) finalAttrs.passthru.tests;
+  buildCommand = ''
+    touch $out
+    for testName in "''${!testResults[@]}"; do
+      if [[ -n "''${testResults[$testName]}" ]]; then
+        echo "$testName success"
+      else
+        echo "$testName fail"
+      fi
+    done
+  ''
+  + lib.optionalString (lib.any (v: !v) (lib.attrValues finalAttrs.testResults)) ''
+    {
+      echo "ERROR: tests.buildenv: Encountering failed tests."
+      for testName in "''${!testResults[@]}"; do
+        if [[ -z "''${testResults[$testName]}" ]]; then
+          echo "- $testName"
+        fi
+      done
+      echo "To inspect the expected and actual result, "
+      echo '  evaluate `tests.buildenv.tests.''${testName}`.'
+    } >&2
+    exit 1
+  '';
+})

pkgs/test/default.nix

@@ -183,6 +183,8 @@ in
 
   nixosOptionsDoc = recurseIntoAttrs (callPackage ../../nixos/lib/make-options-doc/tests.nix { });
 
+  buildenv = callPackage ./buildenv.nix { };
+
   overriding = callPackage ./overriding.nix { };
 
   texlive = recurseIntoAttrs (callPackage ./texlive { });