flydra_analysis: improve tests of FastFinder #181

Job	Run time
test-ubuntu-system-python (ubuntu-20.04)	3m 4s
build (ubuntu-20.04, 3.8)	3m 8s
test-ubuntu-system-python (ubuntu-latest)	2m 49s
build (ubuntu-20.04, 3.x)	3m 1s
build (ubuntu-latest, 3.8)	3m 1s
build (ubuntu-latest, 3.x)	2m 58s
build (macos-11, 3.8)	4m 8s
build (macos-11, 3.x)	4m 49s
build (macos-latest, 3.8)	2m 39s
build (macos-latest, 3.x)	6m 55s
build (windows-2019, 3.8)	5m 25s
build (windows-2019, 3.x)	4m 59s
build (windows-latest, 3.8)	6m 23s
build (windows-latest, 3.x)	6m 25s
	59m 44s

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