TestReline#test_tty_amibuous_width: use test_rubybin

Same as ruby/reline#510, 'ruby' command is not
always available so don't rely on that specific name.

ruby/reline@f60199fed7

Gbp-Pq: Name TestReline-test_tty_amibuous_width-use-test_rubybin.patch

Author: sorah

test/reline/test_reline.rb

@@ -446,7 +446,7 @@ def test_tty_amibuous_width
     RUBY
     ruby_file.close
     lib = File.expand_path('../../lib', __dir__)
-    cmd = [{ 'TERM' => 'xterm' }, 'ruby', '-I', lib, ruby_file.to_path]
+    cmd = [{ 'TERM' => 'xterm' }, Reline.test_rubybin, '-I', lib, ruby_file.to_path]
 
     # Calculate ambiguous width from cursor position
     [1, 2].each do |ambiguous_width|