Tests

[mypyc] Cache built librt in tests (#20626) #20299

Job	Run time
Test mypyc suite with 32-bit Python	9m 20s
Type check our own code (py310-windows-64)	2m 28s
Test suite with py312-ubuntu	10m 18s
Test suite with py311-ubuntu	9m 35s
Test suite with py314-windows-64	27m 19s
Type check our own code (py310-ubuntu)	1m 44s
Parallel tests with py314-ubuntu, mypyc-compiled	33m 14s
Parallel tests with py314-ubuntu, interpreted	24m 45s
Test suite with py313-ubuntu, mypyc-compiled	15m 16s
mypyc runtime tests with py313-macos	4m 16s
Test suite with py314-ubuntu, mypyc-compiled	15m 27s
mypyc runtime tests with py310-ubuntu	5m 21s
Test suite with py310-ubuntu, mypyc-compiled	14m 47s
Formatting and code style with Black + ruff	1m 50s
Test suite with py314t-ubuntu, mypyc-compiled	17m 9s
	3h 12m 49s

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