kernel/generic: add kernelTests automatically

Unfortunately, there seems to be no way of referencing an overridable version of
the package you're in, so it had to be stubed to work at all.

This isn't important for our current, very basic kernel nixosTests but might
become important when we add more sophisticated ones.

pkgs/os-specific/linux/kernel/generic.nix

@@ -58,6 +58,7 @@
 , preferBuiltin ? stdenv.hostPlatform.linux-kernel.preferBuiltin or false
 , kernelArch ? stdenv.hostPlatform.linuxArch
 , kernelTests ? []
+, nixosTests
 , ...
 }:
 
@@ -192,7 +193,16 @@ let
     kernelOlder = lib.versionOlder version;
     kernelAtLeast = lib.versionAtLeast version;
     passthru = kernel.passthru // (removeAttrs passthru [ "passthru" ]);
-    tests = kernelTests;
+    tests = let
+      overridableKernel = finalKernel // {
+        override = args:
+          lib.warn (
+            "override is stubbed for NixOS kernel tests, not applying changes these arguments: "
+            + toString (lib.attrNames (if lib.isAttrs args then args else args {}))
+          ) overridableKernel;
+      };
+    in [ (nixosTests.kernel-generic.testsForKernel overridableKernel) ] ++ kernelTests;
   };
 
-in lib.extendDerivation true passthru kernel
+  finalKernel = lib.extendDerivation true passthru kernel;
+in finalKernel