Create ArtifactPath function to abstract test paths

When AOS is included as an external repo and renamed, paths have to be
updated.  Make a function to handle all this.

Change-Id: If1dfbfb3191809d86da1dca92981b7a0b851c51f
diff --git a/aos/testing/BUILD b/aos/testing/BUILD
index d3287e3..1cff37e 100644
--- a/aos/testing/BUILD
+++ b/aos/testing/BUILD
@@ -117,3 +117,14 @@
         "@com_google_googletest//:gtest",
     ],
 )
+
+cc_library(
+    name = "path",
+    srcs = ["path.cc"],
+    hdrs = ["path.h"],
+    target_compatible_with = ["@platforms//os:linux"],
+    visibility = ["//visibility:public"],
+    deps = [
+        "@com_google_absl//absl/strings",
+    ],
+)