Change debug test workaround to use ENV RUBY_DEBUG_TEST_UI (#1014)

Author: tompng

lib/irb/input-method.rb

@@ -67,7 +67,9 @@ def initialize
 #
 # See IO#gets for more information.
 defgets
-putsif@stdout.tty?# workaround for debug compatibility test
+# Workaround for debug compatibility test https://github.com/ruby/debug/pull/1100
+putsifENV['RUBY_DEBUG_TEST_UI']
+
 print@prompt
 line=@stdin.gets
 @line[@line_no += 1]=line