Initial import of version 0.3.21 am: 991128c83b am: d49f50682b am: bc1346748c am: ace98515a5

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/futures-test/+/2143657

Change-Id: I9a3c12a67fc06ebde75a4405f3b3ca929f99d21e
Signed-off-by: Automerger Merge Worker <[email protected]>