This adds rubocop-rspec, and enables some cops that were either already passing or are passing now, after fixing them in this commit.
Some new cops are disabled for now, with annotation: "TODO" or "To be decided". Those either need to be discussed first, or require manual changes, or the number of found and fixed offenses is too large to bundle them up in a single PR.
Includes:
* DEV: Update rubocop's `TargetRubyVersion` to 2.6
* DEV: Enable RSpec/VoidExpect
* DEV: Enable RSpec/SharedContext
* DEV: Enable RSpec/EmptyExampleGroup (Removed an obsolete empty spec file)
* DEV: Enable RSpec/ItBehavesLike
* DEV: Remove RSpec/ScatteredLet (It's too strict, as it doesn't recognize fab! as a let-like)
* DEV: Remove RSpec/MultipleExpectations
7ff889574d