difftest: nextest support and speedups #718

Job	Run time
Android (ubuntu-24.04, aarch64-linux-android)	8m 7s
Test (macOS-latest)	8m 55s
Compiletest (macOS-latest)	10m 13s
Compiletest (ubuntu-24.04)	8m 40s
Test (windows-2022)	15m 12s
Compiletest (windows-2022)	16m 5s
Difftest (windows-2022)	8m 42s
Difftest (macOS-latest)	8m 53s
Test (ubuntu-24.04)	7m 53s
Difftest (ubuntu-24.04)	8m 56s
Lint	7m 8s
cargo-deny	1m 0s
test_success	4s
	1h 49m 48s

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