Improve testing coverage

[egonw]

Description:

Second, we wish to improve the maintainability of the library and improve the code coverage of
the (unit) testing to at least 50% for all modules (statistics available at app.codecov.io/gh/cdk/cdk).

[egonw]

I had a look at the org.openscience.cdk.controller package (in the extra module), but that one is hard to test, I think: https://app.codecov.io/gh/cdk/cdk/tree/master/misc%2Fcontrol%2Fsrc%2Fmain%2Fjava%2Forg%2Fopenscience%2Fcdk%2Fcontroller

[egonw]

This one worked better. No new code needed, just making more "maven-like": cdk/cdk#1081

Mostly increased counted :)

See https://app.codecov.io/gh/cdk/cdk/pull/1081/indirect-changes

[egonw]

new PR: cdk/cdk#1095