perf: speedup ci by splitting tests into parallel default/all-features jobs

[samlaf]

The test job ran cargo test and cargo test --all-features back to back: 7.8m + 7.7m, about 16m of wall clock. A two-entry matrix runs them side by side and cuts that to about 8m. Each entry gets its own rust-cache shared-key, so the two jobs no longer overwrite each other's cache. fail-fast: false keeps a failure in one config from cancelling the other.

Timings from run 32185252971:

  cargo test                 7.8m   (2m34s compile, ~5m10s tests)
  cargo test --all-features  7.7m   (1m52s compile, ~5m50s tests)

Compilation is only about 30% of the cost. Almost all the rest is one binary: the summit lib test target (94 tests in 307s, 96 tests in 287s). Every other crate finishes in under a second, except summit-syncer under all-features (27 tests, 56s).

Both configs earn their place. --all-features runs a strict superset of the default test names (613 -> 644, and no test is default-only). But three features change behavior in place instead of adding tests:

• bench swaps start_building_block for a variant that takes a height and drops the state root (application/src/actor.rs)
• bad-blocks swaps in BadBlockEngineClient (node/src/args.rs)
• permissioned changes the RPC auth path (rpc/src/server.rs)

bench is on under --all-features, so that run never exercises the production block-building path. The default run is what covers it.

Possible Follow-ups (not done here)

• --all-features turns on bench and bad-blocks together. No deployment runs that combination. A named feature set, such as --features prom,permissioned,e2e, would test a real configuration instead of a frankenconfig.
• To get below ~8m, shard the summit lib binary with cargo nextest run --partition count:N/M over a matrix. The slowest tests are multi-node integration tests. Local timings: 207s for test_checkpoint_verification_fixed_committee, 175s for test_process_time_invalid_new_validator_refund_does_not_merge_with_reused_pubkey_withdrawal, 158s for test_deposit_request_top_up.

[samlaf]

TODO: in order to merge will have to replace the now outdated required CI check "test" and make the 2 new ones requires instead.