[ci] Fix RSpec/FilePath offense

Fixes two types of RSpec/FilePath offenses:
* The RSpec file name should match the one from the tested class.
* RSpec test files have to end with '_spec.rb'. Otherwise rspec does not
  ignores them.

Adding the _spec.rb suffix caused that these files were also recognized
by the RuboCop RSpec cops and RSpec itself.
That's why this commit includes additional RuboCop offense and RSpec
test fixes.