check configs for undefined scenarios

[orichters]

Purpose of this PR

• avoid problems such as bugfix scenario naming in path_gdx column of EL2p0 project config #1817
• check whether all path_gdx* columns are actually defined as a scenario in that file
• do this check only when running make test, because in normal operation mode you might have specified a path to some old run etc. But that should rather not find its way into the repository

Type of change

• New feature

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• I adjusted the reporting in remind2 where it was needed
• I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• All automated model tests pass (FAIL 0 in the output of make test)
• The changelog CHANGELOG.md has been updated correctly

[gabriel-abrahao]

Looks good to me, thanks Oli!