Added workaround for CoreAssertions used by ruby/logger.

test/logger/helper.rb

@@ -3,6 +3,11 @@ $LOAD_PATH.unshift File.join(ROOT_DIR, 'lib') # to use logger in this repo inste
 $LOAD_PATH.unshift File.join(ROOT_DIR, 'test', 'lib') # to use custom test-unit in this repo
 require 'logger'
 require 'test/unit'
-require 'core_assertions'
 
-Test::Unit::TestCase.include Test::Unit::CoreAssertions
+begin
+  # for standalone test suite on ruby/logger
+  require 'core_assertions'
+
+  Test::Unit::TestCase.include Test::Unit::CoreAssertions
+rescue LoadError
+end