diff --git a/tool/lib/core_assertions.rb b/tool/lib/core_assertions.rb
index 4887d944c5..23de1f7e50 100644
--- a/tool/lib/core_assertions.rb
+++ b/tool/lib/core_assertions.rb
@@ -1,6 +1,43 @@
 # frozen_string_literal: true
 
 module Test
+
+  class << self
+    ##
+    # Filter object for backtraces.
+
+    attr_accessor :backtrace_filter
+  end
+
+  class BacktraceFilter # :nodoc:
+    def filter bt
+      return ["No backtrace"] unless bt
+
+      new_bt = []
+      pattern = %r[/(?:lib\/test/|core_assertions\.rb:)]
+
+      unless $DEBUG then
+        bt.each do |line|
+          break if pattern.match?(line)
+          new_bt << line
+        end
+
+        new_bt = bt.reject { |line| pattern.match?(line) } if new_bt.empty?
+        new_bt = bt.dup if new_bt.empty?
+      else
+        new_bt = bt.dup
+      end
+
+      new_bt
+    end
+  end
+
+  self.backtrace_filter = BacktraceFilter.new
+
+  def self.filter_backtrace bt # :nodoc:
+    backtrace_filter.filter bt
+  end
+
   module Unit
     module Assertions
       def assert_raises(*exp, &b)
diff --git a/tool/lib/test/unit.rb b/tool/lib/test/unit.rb
index 68d2ab471e..23dfbd8f0f 100644
--- a/tool/lib/test/unit.rb
+++ b/tool/lib/test/unit.rb
@@ -24,42 +24,6 @@ require 'optparse'
 # See Test::Unit
 module Test
 
-  class << self
-    ##
-    # Filter object for backtraces.
-
-    attr_accessor :backtrace_filter
-  end
-
-  class BacktraceFilter # :nodoc:
-    def filter bt
-      return ["No backtrace"] unless bt
-
-      new_bt = []
-      pattern = %r[/(?:lib\/test/|core_assertions\.rb:)]
-
-      unless $DEBUG then
-        bt.each do |line|
-          break if pattern.match?(line)
-          new_bt << line
-        end
-
-        new_bt = bt.reject { |line| pattern.match?(line) } if new_bt.empty?
-        new_bt = bt.dup if new_bt.empty?
-      else
-        new_bt = bt.dup
-      end
-
-      new_bt
-    end
-  end
-
-  self.backtrace_filter = BacktraceFilter.new
-
-  def self.filter_backtrace bt # :nodoc:
-    backtrace_filter.filter bt
-  end
-
   ##
   # Test::Unit is an implementation of the xUnit testing framework for Ruby.
   module Unit