PSA: `make bench` is changing from `TESTS` to `BENCHES`

I’m about to merge https://github.com/cockroachdb/cockroach/pull/10923 which resolves an issue with our test infrastructure by converging make test and make bench. A consequence of this is that the two targets can’t use the same variable (TESTS) to refer to different things as was the case before.

Going forward, please use make bench BENCHES=... to run benchmarks rather than make bench TESTS=....

Apologies for the inconvenience.