Update dependencies for junit, mockito and checkstyle (#3951)
Update dependencies for junit, mockito and checkstyle (#3951)

Junit 5.1.0 -> 5.1.1
mockito 2.17.0 -> 2.18.0
checkstyle 8.8 -> 8.9
sauliusg/jabref,tobiasdiez/jabref,sauliusg/jabref,tobiasdiez/jabref,zellerdev/jabref,zellerdev/jabref,Siedlerchr/jabref,JabRef/jabref,Siedlerchr/jabref,JabRef/jabref,sauliusg/jabref,tobiasdiez/jabref,zellerdev/jabref,zellerdev/jabref,Siedlerchr/jabref,tobiasdiez/jabref,Siedlerchr/jabref,JabRef/jabref,zellerdev/jabref,sauliusg/jabref,JabRef/jabref