Check `...BACKTRACE` variables gracefully before running tests

[azaslavsky]

This caught me a few times, especially since I had two terminals open, one with export RUST_BACKTRACE=1 and one without. It took me longer than I'd like to admit to figure out why tests were passing in one and not the other!

If this is viewed as a desirable check to keep other contributors from tripping over this quirk, it may be useful to extract into its own utils crate, since the logic of dealing with the interaction of the BLESS and RUST_[LIB_]BACKTRACE` env variables occurs in a number of other internal crates' test suites as well.