Fix faulty corpus transaction detection #1778

Job	Run time
Build Echidna on ubuntu-20.04	3m 37s
Build Echidna on macos-latest	3m 38s
Build Echidna on windows-latest	10m 41s
Test with solc 0.4.25 on ubuntu-latest	3m 16s
Test with solc 0.5.7 on ubuntu-latest	3m 38s
Test with solc 0.6.12 on ubuntu-latest	3m 42s
Test with solc 0.7.5 on ubuntu-latest	3m 57s
Test with solc 0.4.25 on windows-latest	4m 17s
Test with solc 0.5.7 on windows-latest	4m 44s
Test with solc 0.6.12 on windows-latest	5m 38s
Test with solc 0.7.5 on windows-latest	6m 26s
	53m 34s

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