test: replace subprocess servers with in-process threaded servers for coverage tracking #1612

Job	Run time
checks / readme-snippets	14s
checks / pre-commit	34s
checks / test (3.11, lowest-direct, windows-latest)	2m 2s
checks / test (3.10, locked, ubuntu-latest)	46s
checks / test (3.10, locked, windows-latest)	1m 48s
checks / test (3.10, lowest-direct, windows-latest)	1m 40s
checks / test (3.10, lowest-direct, ubuntu-latest)	46s
checks / test (3.12, locked, windows-latest)	1m 59s
checks / test (3.13, locked, windows-latest)	1m 51s
checks / test (3.11, lowest-direct, ubuntu-latest)	45s
checks / test (3.12, locked, ubuntu-latest)	49s
checks / test (3.14, locked, ubuntu-latest)	7m 51s
checks / test (3.11, locked, windows-latest)	1m 42s
checks / test (3.13, lowest-direct, ubuntu-latest)	43s
checks / test (3.14, locked, windows-latest)	8m 1s
checks / test (3.11, locked, ubuntu-latest)	45s
checks / test (3.14, lowest-direct, windows-latest)	7m 48s
checks / test (3.12, lowest-direct, windows-latest)	2m 32s
checks / test (3.14, lowest-direct, ubuntu-latest)	5m 39s
checks / test (3.13, locked, ubuntu-latest)	44s
checks / test (3.12, lowest-direct, ubuntu-latest)	52s
checks / test (3.13, lowest-direct, windows-latest)	1m 44s
all-green	5s
	51m 40s

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