difftest: nextest support and speedups #799

Job	Run time
Android (ubuntu-24.04, aarch64-linux-android)	8m 34s
Test (windows-2022)	17m 12s
Difftest (windows-2022)	18m 6s
Test (ubuntu-24.04)	9m 24s
Compiletest (macOS-latest)	8m 51s
Compiletest (ubuntu-24.04)	6m 42s
Compiletest (windows-2022)	13m 25s
Difftest (ubuntu-24.04)	8m 20s
Difftest (macOS-latest)	10m 12s
Test (macOS-latest)	13m 0s
cargo-deny	1m 5s
Lint	5m 47s
test_success	2s
	2h 0m 40s

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