difftest: nextest support and speedups #716

Job	Run time
Android (ubuntu-24.04, aarch64-linux-android)	8m 17s
Test (macOS-latest)	1m 14s
Difftest (ubuntu-24.04)	1m 21s
Test (windows-2022)	1m 16s
Test (ubuntu-24.04)	1m 6s
Difftest (macOS-latest)	1m 6s
Compiletest (macOS-latest)	9m 4s
Compiletest (ubuntu-24.04)	8m 24s
Lint	6m 25s
Compiletest (windows-2022)	11m 1s
cargo-deny	1m 30s
Difftest (windows-2022)	1m 16s
test_success	2s
	52m 2s

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