test: replace subprocess servers with in-process threaded servers for coverage tracking #1625

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	32s
checks / test (3.11, lowest-direct, ubuntu-latest)	54s
checks / test (3.10, lowest-direct, ubuntu-latest)	58s
checks / test (3.12, lowest-direct, windows-latest)	2m 26s
checks / test (3.10, locked, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, windows-latest)	1m 52s
checks / test (3.10, locked, windows-latest)	1m 28s
checks / test (3.11, locked, ubuntu-latest)	53s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.11, lowest-direct, windows-latest)	1m 44s
checks / test (3.11, locked, windows-latest)	1m 43s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.14, lowest-direct, windows-latest)	6m 42s
checks / test (3.14, locked, ubuntu-latest)	7m 53s
checks / test (3.14, locked, windows-latest)	9m 11s
checks / test (3.13, locked, windows-latest)	2m 28s
checks / test (3.13, lowest-direct, windows-latest)	1m 51s
checks / test (3.12, locked, windows-latest)	1m 58s
checks / test (3.14, lowest-direct, ubuntu-latest)	6m 21s
all-green	2s
	53m 42s

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