Ignore missing historical.mif in tests because it is an optional input file

[dklein-pik]

Purpose of this PR

Ignore missing historical.mif in tests, because we are not allowed to publish the validation.tgz (that contains the historical.mif) to external users, but make test should still pass all tests also for external users.

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

[orichters]

Thanks