Tests

[mypyc] Cache built librt in tests #20297

Job	Run time
Test mypyc suite with 32-bit Python	9m 31s
Test suite with py314-windows-64	27m 16s
Test suite with py314t-ubuntu, mypyc-compiled	18m 56s
Formatting and code style with Black + ruff	1m 56s
Type check our own code (py310-windows-64)	2m 29s
mypyc runtime tests with py310-ubuntu	5m 14s
Test suite with py314-ubuntu, mypyc-compiled	14m 53s
Test suite with py313-ubuntu, mypyc-compiled	15m 33s
Parallel tests with py314-ubuntu, interpreted	24m 51s
Test suite with py310-ubuntu, mypyc-compiled	15m 58s
Parallel tests with py314-ubuntu, mypyc-compiled	30m 19s
Test suite with py311-ubuntu	10m 3s
Type check our own code (py310-ubuntu)	1m 43s
mypyc runtime tests with py313-macos	5m 57s
Test suite with py312-ubuntu	9m 51s
	3h 14m 30s

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