[Rust] Use stable rust for no default test, clean up CI tests

1. Rationale

1. As @jorgecarleitao noted on https://github.com/apache/arrow/pull/9889#discussion_r607720790, we should be running the check if arrow compiles with stable rust as that is what we target for the arrow crate
2. I noticed that there were several redundant settings of `RUSTFLAGS`
3. The titles of many of the tests are confusing (to me) as they have a lot of detailed architecture / rust version information rather than the test title