Implement thorough report source testing ("stress" tests)

Stress tests should be used for all report sources to get better coverage of all provided report elements (columns, filters, conditions) against all supported databases

Columns

• Iteratively adding all report columns
• Applying supported aggregation to each

Filters / Conditions

• Iteratively adding all filters and/or conditions
• Defining test values that activate the filters

Note we already have a suitable testcase for their locaation: https://github.com/moodle/moodle/blob/0f94f2929606559cc7a07387aca1bb0c1030e8d5/reportbuilder/tests/helpers.php

This is something we would use frequently in Workplace, and is a good way to cover various report permutations that throw up errors in specific databases