test: replace subprocess servers with in-process threaded servers for coverage tracking #1622

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	38s
checks / test (3.10, locked, ubuntu-latest)	54s
checks / test (3.10, lowest-direct, ubuntu-latest)	53s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
checks / test (3.10, lowest-direct, windows-latest)	1m 31s
checks / test (3.11, lowest-direct, windows-latest)	1m 53s
checks / test (3.12, lowest-direct, windows-latest)	2m 9s
checks / test (3.12, locked, windows-latest)	2m 1s
checks / test (3.10, locked, windows-latest)	1m 42s
checks / test (3.12, lowest-direct, ubuntu-latest)	58s
checks / test (3.14, lowest-direct, ubuntu-latest)	6m 24s
checks / test (3.13, lowest-direct, windows-latest)	2m 10s
checks / test (3.13, locked, windows-latest)	2m 2s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.14, lowest-direct, windows-latest)	7m 37s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.11, locked, windows-latest)	1m 47s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.14, locked, windows-latest)	8m 39s
checks / test (3.14, locked, ubuntu-latest)	7m 51s
checks / test (3.11, locked, ubuntu-latest)	49s
all-green	3s
	53m 45s

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