Use LaTeX "greater than or equal" symbols instead of >=
Minor improvements to Test Structure section

- Also, add TODOs
Add draft section about what testers should focus on

- This section is experimental and may be removed
Merge 'Best Practices' into 'Structure of Test Directory'
Do not write 'unittest' and 'QtTest' as code snippets when not needed
Close #266 Documentation footer improved

Also see #265.