difftest: nextest support and speedups #724

Job	Run time
cargo-deny	1m 0s
Difftest (windows-2022)	6m 39s
Compiletest (ubuntu-24.04)	8m 38s
Android (ubuntu-24.04, aarch64-linux-android)	8m 25s
Lint	6m 13s
Test (windows-2022)	16m 46s
Test (ubuntu-24.04)	8m 2s
Test (macOS-latest)	8m 18s
Difftest (ubuntu-24.04)	6m 42s
Difftest (macOS-latest)	6m 22s
Compiletest (windows-2022)	15m 59s
Compiletest (macOS-latest)	7m 25s
test_success	2s
	1h 40m 31s

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