Isolate test of schema iteration to determine leak #590

Job	Run time
build_pip (windows-latest, pip)	3m 43s
build_pip (ubuntu-latest, pip)	5m 5s
build_conda (ubuntu-latest, conda)	5m 42s
build_conda (windows-latest, conda)	5m 29s
build_conda (macos-latest, conda)	3m 37s
build_pip (macos-latest, pip)	1m 52s
	25m 28s

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