Simplify saving tx result in coverage (#1158) #1741

Job	Run time
Build Echidna on ubuntu-20.04	3m 30s
Build Echidna on macos-latest	2m 41s
Build Echidna on windows-latest	9m 0s
Test with solc 0.4.25 on ubuntu-latest	3m 38s
Test with solc 0.5.7 on ubuntu-latest	4m 16s
Test with solc 0.6.12 on ubuntu-latest	3m 53s
Test with solc 0.7.5 on ubuntu-latest	4m 18s
Test with solc 0.4.25 on windows-latest	5m 20s
Test with solc 0.5.7 on windows-latest	5m 1s
Test with solc 0.6.12 on windows-latest	5m 26s
Test with solc 0.7.5 on windows-latest	6m 10s
	53m 13s

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