tests: support not installing extra packages for in-vm kernel tests

This makes sense for testing if in-vm kernel works out of the box in
selected templates.

QubesOS/qubes-issues#5212

qubes/tests/integ/grub.py

@@ -46,6 +46,11 @@ def setUp(self):
             )
 
     def install_packages(self, vm):
+        if os.environ.get("QUBES_TEST_SKIP_KERNEL_INSTALL") == "1":
+            return
+        else:
+            print("Installing kernel packages, you can skip by setting "
+                  "QUBES_TEST_SKIP_KERNEL_INSTALL=1 in environment")
         if self.template.startswith("fedora-"):
             cmd_install1 = (
                 "dnf clean expire-cache && "