(maint) Ensure get_acceptable_puppet_run_exit_codes emits an array

Previously if none of the catch_* and expect_* options where set the
get_acceptable_puppet_run_exit_codes method would emit a Range (0..256).  However
this raised errors in the handle_puppet_run_returned_exit_code method because
you cannot do a `.join` on a Range.  This commit instead always outputs an
array type from get_acceptable_puppet_run_exit_codes.

lib/beaker/testmode_switcher/runner_base.rb

@@ -23,7 +23,7 @@ def get_acceptable_puppet_run_exit_codes(opts = {})
 
         # If no option supplied, return all exit codes, as an array,
         # as acceptable so beaker returns a detailed output
-        (0...256)
+        (0...256).map { |i| i }
       end
 
       def handle_puppet_run_returned_exit_code(acceptable_exit_codes, returned_exit_code)