test: replace subprocess servers with in-process threaded servers for coverage tracking #1629

Job	Run time
checks / readme-snippets	14s
checks / pre-commit	40s
checks / test (3.11, locked, ubuntu-latest)	48s
checks / test (3.10, locked, windows-latest)	1m 30s
checks / test (3.10, lowest-direct, windows-latest)	1m 37s
checks / test (3.12, locked, windows-latest)	2m 20s
checks / test (3.12, locked, ubuntu-latest)	52s
checks / test (3.10, locked, ubuntu-latest)	52s
checks / test (3.11, locked, windows-latest)	1m 34s
checks / test (3.12, lowest-direct, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.13, lowest-direct, windows-latest)	2m 4s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.14, lowest-direct, ubuntu-latest)	6m 42s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.13, locked, windows-latest)	1m 54s
checks / test (3.14, locked, windows-latest)	8m 16s
checks / test (3.11, lowest-direct, windows-latest)	1m 47s
checks / test (3.12, lowest-direct, windows-latest)	2m 9s
checks / test (3.14, locked, ubuntu-latest)	7m 29s
checks / test (3.14, lowest-direct, windows-latest)	7m 18s
all-green	5s
	52m 38s

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