[gyb] Add gyb doctest to validation-test

Author: rintaro

validation-test/Python/gyb.swift

@@ -0,0 +1,4 @@
+// RUN: %{python} -m doctest %S/../../utils/gyb.py
+// RUN: echo 'Hello ${ME}' | %S/../../utils/gyb --test -DME=Swift | FileCheck %s
+// RUN: echo 'Hello ${ME}' | %S/../../utils/gyb --verbose-test -DME=Swift | FileCheck %s
+// CHECK: Hello Swift