Add rust_test and update TEST_MAPPING am: 99abb8cc83 am: 52e038d115 am: 579c369b07

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-util/+/1366083

Change-Id: Ida28402cc218c1ac6de7adfc5403a99bd40f0857