Tests

Fix sequential bottleneck in parallel parsing (#21291) #21864

Job	Run time
Test mypyc suite with 32-bit Python	10m 45s
Test suite with py314-windows-64	25m 17s
Parallel tests with py314-ubuntu, mypyc-compiled	26m 35s
Test suite with py313-ubuntu, mypyc-compiled	14m 53s
Parallel tests with py314-windows-64, interpreted	8m 5s
Test suite with py311-ubuntu	9m 3s
Test suite with py314t-ubuntu, mypyc-compiled	16m 29s
Parallel tests with py314-ubuntu, interpreted	25m 29s
mypyc runtime tests with py313-macos	6m 27s
Test suite with py312-ubuntu	9m 39s
mypyc runtime tests with py310-ubuntu	5m 29s
Type check our own code (py310-ubuntu)	1m 5s
Type check our own code (py310-windows-64)	2m 2s
Test suite with py314-ubuntu, mypyc-compiled	15m 32s
Test suite with py310-ubuntu, mypyc-compiled	17m 10s
Formatting and code style with Black + ruff	1m 59s
	3h 15m 59s

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