Use Kestrel for all in-memory HTTP tests #482

Job	Run time
build (windows-latest, Release)	4m 37s
build (windows-latest, Debug)	4m 40s
build (ubuntu-latest, Release)	2m 2s
build (ubuntu-latest, Debug)	2m 8s
build (macos-latest, Release)	1m 3s
build (macos-latest, Debug)	1m 2s
publish-coverage / publish-coverage	37s
	16m 9s

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