New cop to discourage `context` with only a single test

Relevant to #2000.

When the `context` block has only a single test, it makes little sense to actually use the `context` block and making the code unnecessary indented and a little more complicated.

```ruby
# bad
context 'when condition' do
  it 'tests something' do
  end
end

# good
it 'tests something when condition' do
end
```