difftest: nextest support and speedups #719

Job	Run time
Compiletest (ubuntu-24.04)	8m 30s
Compiletest (macOS-latest)	7m 35s
Android (ubuntu-24.04, aarch64-linux-android)	8m 5s
Difftest (windows-2022)	11m 56s
Test (ubuntu-24.04)	7m 57s
Compiletest (windows-2022)	16m 21s
Difftest (ubuntu-24.04)	12m 2s
Test (macOS-latest)	9m 18s
cargo-deny	1m 19s
Test (windows-2022)	15m 43s
Lint	7m 14s
Difftest (macOS-latest)	12m 7s
test_success	3s
	1h 58m 10s

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