pkgs/tests: init for testing build of unexposed packages

(cherry picked from commit 34d14bd)

flake.nix

@@ -42,6 +42,7 @@
             constituents = builtins.attrValues self.packages.${system};
           };
         }
+        // (import ./pkgs/tests {inherit pkgs self;})
         // (import ./ioc/tests {inherit pkgs self;})
         // (import ./nixos/tests/all-tests.nix {inherit nixpkgs pkgs self system;});
 

pkgs/tests/default.nix

@@ -0,0 +1,8 @@
+# For packages that are not directly exposed to the user,
+# but should still work.
+#
+# For example kernel modules, which depend on the kernel version,
+# or Python libraries, which depend on the Python version.
+{pkgs, ...}: {
+  mrf-driver-default-linux = pkgs.linuxPackages.mrf;
+}