Compiletest: pass `-Zui-testing` to rustc #1102

Job	Run time
Difftest (ubuntu-24.04)	16m 33s
Difftest (windows-2022)	27m 15s
Android (ubuntu-24.04, aarch64-linux-android)	8m 33s
Compiletest (ubuntu-24.04)	6m 57s
Lint	6m 2s
Difftest (macOS-latest)	21m 22s
Compiletest (macOS-latest)	7m 36s
Compiletest (windows-2022)	15m 15s
cargo-deny	1m 29s
Test (macOS-latest)	16m 59s
Test (ubuntu-24.04)	11m 6s
Test (windows-2022)	19m 21s
test_success	2s
	2h 38m 30s

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