For RUST-1674, we switched to using cargo-nextest as the test runner; this had the unfortunate side effect of making all tests run in sequence. We should be able to use per-test overrides to regain parallelism by default. As a first sketch:

• Update nextest.toml to set test-threads to "num-cpus"
• Add an override section that matches on test names starting with exclusive_ that sets threads-required to "num-cpus"
• Update all current tests using LOCK.run_exclusively to instead have the exclusive_ name prefix.

This does introduce magic name-dependent behavior; there may be a better way.