nixos/testing: Extract nixos-test-base.nix NixOS module

nixos/lib/testing/nixos-test-base.nix

@@ -0,0 +1,23 @@
+# A module containing the base imports and overrides that
+# are always applied in NixOS VM tests, unconditionally,
+# even in `inheritParentConfig = false` specialisations.
+{ lib, ... }:
+let
+  inherit (lib) mkForce;
+in
+{
+  imports = [
+    ../../modules/virtualisation/qemu-vm.nix
+    ../../modules/testing/test-instrumentation.nix # !!! should only get added for automated test runs
+    { key = "no-manual"; documentation.nixos.enable = false; }
+    {
+      key = "no-revision";
+      # Make the revision metadata constant, in order to avoid needless retesting.
+      # The human version (e.g. 21.05-pre) is left as is, because it is useful
+      # for external modules that test with e.g. testers.nixosTest and rely on that
+      # version number.
+      config.system.nixos.revision = mkForce "constant-nixos-revision";
+    }
+
+  ];
+}