Tests

Add hidden --overwrite-union-syntax option #19571

Job	Run time
Test mypyc suite with 32-bit Python	9m 8s
Test suite with py310-ubuntu, mypyc-compiled	13m 51s
Test suite with py311-ubuntu	9m 24s
mypyc runtime tests with py313-macos	5m 56s
Test suite with py314-ubuntu, mypyc-compiled	13m 50s
Type check our own code (py310-ubuntu)	1m 30s
Test suite with py312-ubuntu	9m 59s
mypyc runtime tests with py310-ubuntu	5m 1s
Test suite with py314-windows-64	13m 46s
Formatting and code style with Black + ruff	1m 55s
Test suite with py313-ubuntu, mypyc-compiled	13m 45s
Type check our own code (py310-windows-64)	2m 23s
	1h 40m 28s

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