test: Fix ENV.cxx11

ENV.cxx11 in a test block fails on Linux with
Error: ibex: failed
An exception occurred within a child process:
NoMethodError: undefined method `non_apple_gcc_version'
See for example brew test ibex on Linux.

Library/Homebrew/extend/ENV/shared.rb

@@ -320,7 +320,7 @@ module SharedEnvExtension
     return if compiler_any_clang?(cc)
 
     version = if cc == :gcc
-      non_apple_gcc_version "gcc"
+      DevelopmentTools.non_apple_gcc_version "gcc"
     else
       cc[/^gcc-(\d+(?:\.\d+)?)$/, 1]
     end