Plotting test case improvements

Various improvements to plotting test case:

• Put artifacts are in an additional folder.
• Only upload artifacts if they are different from their reference commit, otherwise refer to reference commit.
• Read the environment variable referenceCommit_$branchName (where $branchName is the name of the git branch, in which occurrences of - are replaced by _) and compare CI plots to the commit specified in the variable. The variable can be set in gitlab.
• Fix small inconsistencies with test tags.