Tests

Add hidden --overwrite-union-syntax option #19621

Job	Run time
Test mypyc suite with 32-bit Python	9m 17s
Test suite with py313-ubuntu, mypyc-compiled	14m 13s
Test suite with py310-ubuntu, mypyc-compiled	14m 27s
Test suite with py312-ubuntu	9m 49s
Parallel tests with py314-ubuntu, mypyc-compiled	27m 0s
Test suite with py311-ubuntu	8m 51s
mypyc runtime tests with py310-ubuntu	5m 0s
Type check our own code (py310-windows-64)	2m 11s
Formatting and code style with Black + ruff	2m 2s
Type check our own code (py310-ubuntu)	1m 38s
Test suite with py314-windows-64	24m 28s
mypyc runtime tests with py313-macos	6m 1s
Parallel tests with py314-ubuntu, interpreted	23m 0s
Test suite with py314-ubuntu, mypyc-compiled	14m 13s
	2h 42m 10s

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