Allow installing unit tests

Closes #9343

See that issue for motivation.

Installing these is disabled by default, but we enable it (and the
additional output we want isntall these too so as not to clutter the
existing ones) to use in cross builds and dev shells.

flake.nix

@@ -164,6 +164,10 @@
 
         testConfigureFlags = [
           "RAPIDCHECK_HEADERS=${lib.getDev rapidcheck}/extras/gtest/include"
+        ] ++ lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
+          "--enable-install-unit-tests"
+          "--with-check-bin-dir=${builtins.placeholder "check"}/bin"
+          "--with-check-lib-dir=${builtins.placeholder "check"}/lib"
         ];
 
         internalApiDocsConfigureFlags = [
@@ -404,7 +408,8 @@
             src = nixSrc;
             VERSION_SUFFIX = versionSuffix;
 
-            outputs = [ "out" "dev" "doc" ];
+            outputs = [ "out" "dev" "doc" ]
+              ++ lib.optional (currentStdenv.hostPlatform != currentStdenv.buildPlatform) "check";
 
             nativeBuildInputs = nativeBuildDeps;
             buildInputs = buildDeps
@@ -710,7 +715,8 @@
           stdenv.mkDerivation {
             name = "nix";
 
-            outputs = [ "out" "dev" "doc" ];
+            outputs = [ "out" "dev" "doc" ]
+              ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) "check";
 
             nativeBuildInputs = nativeBuildDeps
               ++ lib.optional stdenv.cc.isClang pkgs.buildPackages.bear