qtestlib doc: create a section on common patterns and good practices

Auto tests in Qt have been written with various flavours of qtestlib, with the same test functions mixing several styles. Subsequent tests are frequently added by copy-and-paste from other tests, including bad practices. Over the years this have resulted in non-optimal and flakey tests. Create a section in docs on common patterns and good practices. This page later could easily be extended when discovering other anti-patterns found during fixing flakey and other odd looking tests.

Reduce fragmentation by merging https://wiki.qt.io/Unit_Testing and https://wiki.qt.io/Writing_Unit_Tests into this official documentation. Wiki pages is a lazy way of documenting things properly. Note: the contents of these wiki pages needs to be evaluated when moving to the official documentation.

There should also be a subsection on debugging techniques. See https://codereview.qt-project.org/#/c/208303/5/src/testlib/qtesthelpers_p.h for an example of what this section could contain.

End result of this task would be a single point of a reference for new contributions of tests.