difftest: nextest support and speedups #730

Job	Run time
Difftest (ubuntu-24.04)	7m 51s
Difftest (windows-2022)	16m 38s
Lint	7m 16s
Difftest (macOS-latest)	6m 35s
cargo-deny	55s
Test (ubuntu-24.04)	8m 15s
Android (ubuntu-24.04, aarch64-linux-android)	8m 8s
Test (windows-2022)	14m 34s
Compiletest (windows-2022)	15m 47s
Test (macOS-latest)	9m 27s
Compiletest (macOS-latest)	9m 32s
Compiletest (ubuntu-24.04)	9m 41s
test_success	2s
	1h 54m 41s

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