Add type annotations to `test-data/unit/plugins` #10998

Job	Run time
Test mypyc suite with 32-bit Python	10m 33s
Test suite with py38-ubuntu, mypyc-compiled	28m 35s
Test suite with py38-windows-64	33m 14s
Test suite with py39-ubuntu	27m 29s
Test suite with py310-ubuntu	26m 17s
Test suite with py311-ubuntu, mypyc-compiled	20m 38s
Test suite with py312-ubuntu, mypyc-compiled	28m 3s
mypyc runtime tests with py38-macos	6m 46s
mypyc runtime tests with py38-debug-build-ubuntu	8m 51s
Type check our own code (py38-ubuntu)	2m 9s
Type check our own code (py38-windows-64)	3m 20s
Formatting and code style with Black + ruff	40s
	3h 16m 35s

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