Update Truffle strscan test to use ruby_exe.

Author: aardvark179

spec/truffle/strscan/strscan_spec.rb

@@ -4,6 +4,6 @@
 
 describe 'StringScanner' do
   it 'has a `scan_internal` method of under 100 AST nodes' do
-    Truffle::Debug.ast_size(StringScanner.instance_method(:scan_internal)).should < 100
+    ruby_exe('require "strscan"; print Truffle::Debug.ast_size(StringScanner.instance_method(:scan_internal))').to_i.should < 100
   end
 end