[ci skip] Clarify some Minitest origins.

It might be tough for readers to know why we implement `===`, and where
the Regexp in `derive_regexp` came from.

railties/lib/rails/test_unit/line_filtering.rb

@@ -18,12 +18,14 @@ def initialize(runnable, filter, patterns)
       @filters = [ derive_regexp(filter), *derive_line_filters(patterns) ].compact
     end
 
+    # Minitest uses === to find matching filters.
     def ===(method)
       @filters.any? { |filter| filter === method }
     end
 
     private
       def derive_regexp(filter)
+        # Regexp filtering copied from Minitest.
         filter =~ %r%/(.*)/% ? Regexp.new($1) : filter
       end