Tests

Fixes #15127: Added optional error code to flag missing return type and tests #18618

Job	Run time
Test mypyc suite with 32-bit Python	8m 36s
Test suite with py312-ubuntu, mypyc-compiled	13m 4s
Test suite with py39-ubuntu, mypyc-compiled	15m 29s
Test suite with py311-ubuntu	8m 17s
Formatting and code style with Black + ruff	1m 59s
Test suite with py314-dev-ubuntu	16m 34s
Type check our own code (py39-ubuntu)	1m 34s
Test suite with py313-ubuntu, mypyc-compiled	13m 58s
mypyc runtime tests with py39-macos	5m 52s
Type check our own code (py39-windows-64)	2m 27s
Test suite with py313-windows-64	25m 41s
Test suite with py310-ubuntu	10m 42s
	2h 4m 13s

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