Fix -Xembedded spec to allow warnings messages on STDERR

(cherry picked from commit 12a13eb)

Author: eregon

spec/truffle/options/embedded_spec.rb

@@ -34,6 +34,6 @@
   end
 
   it "when enabled will warn about signals" do
-    ruby_exe("Signal.trap('ALRM') { }", options: "-Xembedded").should =~ /trapping signal ALRM in embedded mode/
+    ruby_exe("Signal.trap('ALRM') { }", options: "-Xembedded", args: "2>&1").should include("trapping signal ALRM in embedded mode")
   end
 end