Fix MonitoringCoreKernelTest::testImageMissingStyleSensorPlugin

Problem/Motivation

From #2648174: Update MonitoringSearchAPITest #7 we got an odd failing test. The test comes from #2610132: Add unaggregated verbose output of image derivative creation sensor. We have tested it manually and it works fine, but somehow it does not work on this test.

Seems that when WatchdogAggregatorSensorPlugin in the verboseResultUnaggregated, tries to add the link to the details for the column "wid" the method Url::fromUserInput throws a DatabaseException with the message: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'd8.simpletest376050router' doesn't exist: DELETE FROM {router}

The event is added in the same way for the demo, and it works fine there.
So we have to find why it doesn't work in the test.

Proposed resolution

- find the problem
- fix it

Remaining tasks

User interface changes

API changes

Data model changes