test: replace subprocess servers with in-process threaded servers for coverage tracking #1626

Job	Run time
checks / pre-commit	35s
checks / readme-snippets	14s
checks / test (3.10, lowest-direct, windows-latest)	1m 51s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.11, locked, windows-latest)	1m 44s
checks / test (3.11, lowest-direct, windows-latest)	1m 43s
checks / test (3.12, locked, windows-latest)	1m 56s
checks / test (3.12, lowest-direct, ubuntu-latest)	59s
checks / test (3.10, locked, windows-latest)	1m 35s
checks / test (3.13, locked, windows-latest)	1m 44s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.12, locked, ubuntu-latest)	58s
checks / test (3.13, lowest-direct, windows-latest)	3m 20s
checks / test (3.14, lowest-direct, ubuntu-latest)	6m 22s
checks / test (3.12, lowest-direct, windows-latest)	2m 13s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.14, locked, ubuntu-latest)	7m 53s
checks / test (3.14, locked, windows-latest)	8m 46s
checks / test (3.14, lowest-direct, windows-latest)	7m 21s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
all-green	3s
	54m 24s

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