Build test indexes in parallel using `rayon`. #3037

Job	Run time
Build binary	39s
Build binary (Windows)	2m 38s
Semver: core-graphics	59s
Check lint and rustfmt	33s
Run tests (1.80, false)	2m 40s
Check if version changed	19s
Run tests (1.81, false)	2m 49s
Run tests (1.82, false)	2m 43s
Run tests (stable, false)	2m 39s
Run tests (beta, false)	2m 38s
Run tests (nightly, true)	2m 32s
Caching with feature config changed	29s
Caching across rustdoc versions	1m 31s
Semver: libp2p-gossipsub ~0.44.0 + libp2p-request-response ~0.24.0	1m 20s
Semver: libp2p-dcutr 1.62.0 and libp2p-relay 0.14.0	1m 14s
Semver: libp2p-core 0.37.0	58s
Semver: bevy_core v0.9.0	42s
Semver: bevy_gltf v0.9.1	1m 3s
Semver: clap v3.2.0	52s
Semver: libp2p-uds 0.37.0	56s
Semver: tokio ~v1.25.0 (explicit)	2s
Semver: sqllogictest v0.13.0	37s
Semver: tokio ~v1.25.0 (implicit)	5s
Semver: dependency has warnings with "-Dwarnings" (libp2p v0.53.1)	1m 19s
Semver: perf-event-open-sys2 v5.0.0	18s
Check failure when installed rustc is too old	21s
Semver: ref-slice fork (git)	40s
Semver: ref-slice fork (sparse)	29s
Semver: ref-slice fork (Windows) (git)	1m 7s
Semver: ref-slice fork (Windows) (sparse)	48s
All CI stages	1s
Check for semver compliance	0s
Run the release workflow	0s
	36m 1s

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