Improve tests around stopFetches and max fetches per host. #330

Job	Run time
swift (debug)	3m 59s
xcodebuild (ios, Debug)	10m 36s
swift (release)	3m 56s
xcodebuild (ios, Release)	10m 35s
xcodebuild (macos, Debug)	5m 0s
xcodebuild (macos, Release)	4m 53s
xcodebuild (tvos, Debug)	5m 33s
xcodebuild (tvos, Release)	6m 38s
xcodebuild (visionos, Debug)	6m 9s
xcodebuild (visionos, Release)	4m 56s
xcodebuild (watchos, Debug)	1m 4s
xcodebuild (watchos, Release)	53s
	1h 4m 12s

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