Simplify normalization of regexp test filters

Follow-up to rails#47942.

This commit simplifies the normalization of regexp test filters.
Instead of modifying a given regexp to match a union of whitespace and
underscores, the regexp is unioned with the underscore-normalized
version of itself.  This allows filters that include escaped spaces,
such as `/foo\ bar/`.

This commit also fixes `Rails::LineFiltering#run` such that
normalization isn't reapplied for every test suite.  Thus
`normalize_declarative_test_filter` is no longer required to be
idempotent.

Co-authored-by: Jonathan Hefner <[email protected]>

railties/lib/rails/test_unit/line_filtering.rb

@@ -5,7 +5,7 @@
 module Rails
   module LineFiltering # :nodoc:
     def run(reporter, options = {})
-      options[:filter] = Rails::TestUnit::Runner.compose_filter(self, options[:filter])
+      options = options.merge(filter: Rails::TestUnit::Runner.compose_filter(self, options[:filter]))
 
       super
     end

railties/lib/rails/test_unit/runner.rb

@@ -52,7 +52,7 @@ def load_tests(argv)
         end
 
         def compose_filter(runnable, filter)
-          filter = escape_declarative_test_filter(filter)
+          filter = normalize_declarative_test_filter(filter)
 
           if filters.any? { |_, lines| lines.any? }
             CompositeFilter.new(runnable, filter, filters)
@@ -104,12 +104,12 @@ def list_tests(patterns)
             tests
           end
 
-          def escape_declarative_test_filter(filter)
-            # NOTE: This method may be applied multiple times, so any
-            # transformations MUST BE idempotent.
+          def normalize_declarative_test_filter(filter)
             if filter.is_a?(String)
               if regexp_filter?(filter)
-                filter = filter.gsub(/\s+/, '[\s_]+')
+                # Minitest::Spec::DSL#it does not replace whitespace in method
+                # names, so match unmodified method names as well.
+                filter = filter.gsub(/\s+/, "_").delete_suffix("/") + "|" + filter.delete_prefix("/")
               elsif !filter.start_with?("test_")
                 filter = "test_#{filter.gsub(/\s+/, "_")}"
               end

railties/test/application/test_runner_test.rb

@@ -693,7 +693,7 @@ class PostTest < ActiveSupport::TestCase
         end
       RUBY
 
-      run_test_command("test/models/post_test.rb -n '/greets foo|greets .  . bar/'").tap do |output|
+      run_test_command("test/models/post_test.rb -n '/greets foo|greets .  .\\ bar/'").tap do |output|
         assert_match "hello foo", output
         assert_match "hello again foo", output
         assert_match "hello bar", output
@@ -703,9 +703,12 @@ class PostTest < ActiveSupport::TestCase
 
     def test_declarative_style_regexp_filter_with_minitest_spec
       app_file "test/models/post_test.rb", <<~RUBY
+        require "test_helper"
         require "minitest/spec"
 
-        class PostTest < Minitest::Spec
+        class PostTest < ActiveSupport::TestCase
+          extend Minitest::Spec::DSL
+
           it "greets foo" do
             puts "hello foo"
             assert true
@@ -727,7 +730,7 @@ class PostTest < Minitest::Spec
         end
       RUBY
 
-      run_test_command("test/models/post_test.rb -n '/greets foo|greets .  . bar/'").tap do |output|
+      run_test_command("test/models/post_test.rb -n '/greets foo|greets .  .\\ bar/'").tap do |output|
         assert_match "hello foo", output
         assert_match "hello again foo", output
         assert_match "hello bar", output