test that manipulateConfig does not destroy comments and can undo changes

[orichters]

Purpose of this PR

• with adjust variable = value regexp in manipulateConfig to avoid false positives in comments pik-piam/lucode2#205, the destruction of comments has ended, as well as whitespace changes
• therefore: remove all exceptions from the test such that also comment changes are tracked
• additionally, overwrite the tmpfile with some rather arbitrary data and check whether reading that again works
• then reset it to the default and check that nothing changes
• make sclass call conditional on existence

Type of change

• Bug fix

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): [ FAIL 0 | WARN 0 | SKIP 6 | PASS 88 ]
• The changelog CHANGELOG.md has been updated correctly