Improve `declare_class!` tests #2082

Job	Run time
Check formatting	8s
Check MSRV	1m 51s
Lint macOS 64bit	5m 34s
Lint iOS 32bit	1m 43s
Lint GNUStep + exceptions	1m 40s
Lint header-translator	42s
Lint test-assembly	43s
Test macOS 12	7m 26s
Verify header translator output	1m 16s
Compiler UI	1m 13s
Assembly tests	6m 9s
Test Compiler-RT	27s
Check icrate features	0s
${{ matrix.name }}	0s
Test GNUStep with libobjc2 v1.9	1m 59s
Test GNUStep with libobjc2 v2.0	1m 58s
Test GNUStep with libobjc2 v2.1 on nightly	2m 6s
Test GNUStep 32bit	2m 13s
Test on iOS simulator w. dinghy	0s
	37m 8s

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