difftest: nextest support and speedups #732

Job	Run time
Android (ubuntu-24.04, aarch64-linux-android)	7m 56s
Compiletest (ubuntu-24.04)	8m 19s
Compiletest (macOS-latest)	6m 23s
Test (ubuntu-24.04)	7m 48s
Test (macOS-latest)	6m 52s
Compiletest (windows-2022)	8m 57s
Test (windows-2022)	8m 58s
Difftest (windows-2022)	8m 58s
cargo-deny	1m 28s
Difftest (ubuntu-24.04)	9m 2s
Lint	6m 12s
Difftest (macOS-latest)	8m 57s
test_success	3s
	1h 29m 53s

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