remove testing from concepts.csv
remove testing from concepts.csv

Test driven development is out of scope for the Rust track.
See
https://github.com/exercism/v3/blob/master/languages/reference/out-of-scope.md
for other examples.

exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust