Infrastructure for running tests under Linux in primary VM.

Includes an initial test of simply inserting and removing the Hafnium
kernel module.

Change-Id: I832a30d902f58ca71f89374300ab39b2ba3ab877
diff --git a/BUILD.gn b/BUILD.gn
index 3cbad6e..21d1869 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,3 +30,17 @@
     "//project/${project}:test_root",
   ]
 }
+
+group("update_prebuilts") {
+  deps = [
+    "//third_party:linux",
+  ]
+}
+
+group("default") {
+  testonly = true
+  deps = [
+    ":root",
+    ":test_root",
+  ]
+}