difftest: nextest support and speedups #1326

Job	Run time
Android (ubuntu-latest, aarch64-linux-android)	8m 41s
Difftest (ubuntu-latest)	9m 43s
Difftest (windows-latest)	17m 42s
Difftest (macOS-latest)	8m 51s
Compiletest (macOS-latest)	10m 26s
Test (windows-latest)	20m 13s
Lint	6m 0s
Test (ubuntu-latest)	11m 7s
Compiletest (windows-latest)	18m 25s
Compiletest (ubuntu-latest)	7m 51s
cargo-deny	1m 32s
Test (macOS-latest)	14m 53s
test_success	3s
	2h 15m 27s

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