difftest: nextest support and speedups #725

Job	Run time
Android (ubuntu-24.04, aarch64-linux-android)	7m 48s
Difftest (macOS-latest)	10m 29s
Test (windows-2022)	15m 22s
Test (ubuntu-24.04)	8m 28s
Compiletest (macOS-latest)	7m 53s
Test (macOS-latest)	9m 54s
Lint	6m 25s
Difftest (windows-2022)	11m 57s
Compiletest (windows-2022)	15m 47s
Compiletest (ubuntu-24.04)	8m 25s
cargo-deny	1m 4s
Difftest (ubuntu-24.04)	8m 8s
test_success	3s
	1h 51m 43s

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