testing/test-instrumentation: Decrease stateVersion priority

Set the priority of `system.stateVersion` between `mkDefault` and
`mkOptionDefault`.
Since this is set unconditionally for test instrumentation purposes, and
it is specifically set to:
```
Squelch warning about unset system.stateVersion
```
it should be no problem to not conflict if someone if setting this
option as `mkDefault`.

nixos/modules/testing/test-instrumentation.nix

@@ -239,7 +239,7 @@ in
     services.qemuGuest.package = pkgs.qemu_test.ga;
 
     # Squelch warning about unset system.stateVersion
-    system.stateVersion = lib.mkDefault lib.trivial.release;
+    system.stateVersion = (lib.mkOverride 1200) lib.trivial.release;
   };
 
 }