difftest: nextest support and speedups #720

Job	Run time
Compiletest (macOS-latest)	6m 3s
Compiletest (ubuntu-24.04)	6m 5s
Difftest (ubuntu-24.04)	1m 57s
Test (macOS-latest)	6m 1s
Android (ubuntu-24.04, aarch64-linux-android)	6m 6s
Test (windows-2022)	6m 0s
Test (ubuntu-24.04)	6m 6s
Difftest (windows-2022)	1m 41s
Lint	6m 6s
cargo-deny	55s
Compiletest (windows-2022)	5m 58s
Difftest (macOS-latest)	1m 37s
test_success	4s
	54m 39s

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