Suppress false positives of RSpec/RedundantPredicateMatcher

This commit suppresses the following false positives of `RSpec/RedundantPredicateMatcher`:

```console
$ bundle exec rubocop

Offenses:

spec/rubocop/ast/processed_source_spec.rb:418:41: C: [Correctable] RSpec/RedundantPredicateMatcher:
Use start_with instead of be_start_with.
        expect(processed_source).not_to be_start_with('start')
                                        ^^^^^^^^^^^^^^^^^^^^^^
(snip)
```

`ProcessedSource` has `start_with?` method, and `be_start_with` is used as a predicate matcher for it.

spec/rubocop/ast/processed_source_spec.rb

@@ -410,6 +410,7 @@ def some_method
     end
   end
 
+  # rubocop:disable RSpec/RedundantPredicateMatcher
   describe '#start_with?' do
     context 'with blank source' do
       let(:source) { '' }
@@ -439,6 +440,7 @@ def some_method
       end
     end
   end
+  # rubocop:enable RSpec/RedundantPredicateMatcher
 
   describe '#preceding_line' do
     let(:source) { <<~RUBY }