Introduce artifact_path() for rust

This makes it so that rust tests inside of AOS do not have to care about
whether the repository is named "aos", "org_frc971", etc. This also
takes the opportunity to fix up the C++ equivalent to not even have the
repo name hard-coded in the library.

Change-Id: Ie3114c6a420fa46c1cc0f7acfe8115232b10eae3
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/BUILD b/aos/BUILD
index e67b593..880985f 100644
--- a/aos/BUILD
+++ b/aos/BUILD
@@ -346,6 +346,9 @@
     test_data = [
         "//aos/testdata:test_configs",
     ],
+    test_deps = [
+        "//aos/testing:path_rs",
+    ],
     visibility = ["//visibility:public"],
     deps = [
         ":configuration_rust_fbs",