Use Kestrel for all in-memory HTTP tests #491

Job	Run time
build (windows-latest, Release)	5m 4s
build (ubuntu-latest, Debug)	3m 58s
build (ubuntu-latest, Release)	2m 2s
build (macos-latest, Debug)	59s
build (windows-latest, Debug)	5m 4s
build (macos-latest, Release)	1m 30s
publish-coverage / publish-coverage	38s
	19m 15s

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