Allow a regex filter for RUST_LOG

When specifying RUST_LOG, the programmer may append `/regex` to the end of the spec. All results will then be filtered using that regex.
auto merge of #16553 : nick29581/rust/log, r=huon

When specifying RUST_LOG, the programmer may append `/regex` to the end of the spec. All results will then be filtered using that regex.

r?