Allow test cases to inject config parameters into the config

[white-gecko]

This commits enables designers of test cases to inject config parameters into the config, e.g. for disabling caching or other features.

[pfrischmuth]

• [x] manual review commits
• [x] check unit tests: Tests: Tests: 648, Assertions: 1294, Incomplete: 3, Skipped: 8.
• [x] check integration tests mysql: Tests: 63, Assertions: 5834, Skipped: 7.
• [x] check integration tests virtuoso: Tests: 63, Assertions: 224, Skipped: 9.

[pfrischmuth]

OK