Tests

Add hidden --overwrite-union-syntax option #19572

Job	Run time
Test mypyc suite with 32-bit Python	9m 14s
Test suite with py310-ubuntu, mypyc-compiled	14m 57s
Test suite with py311-ubuntu	9m 6s
Test suite with py312-ubuntu	9m 59s
Test suite with py314-ubuntu, mypyc-compiled	14m 26s
mypyc runtime tests with py310-ubuntu	5m 0s
Type check our own code (py310-windows-64)	2m 26s
Type check our own code (py310-ubuntu)	1m 35s
Test suite with py314-windows-64	26m 58s
mypyc runtime tests with py313-macos	4m 20s
Formatting and code style with Black + ruff	1m 52s
Test suite with py313-ubuntu, mypyc-compiled	14m 16s
	1h 54m 9s

Provide feedback