test_gpg: add initial tests

Library/Homebrew/test/test_gpg.rb

+require "testing_env"
+require "gpg"
+
+class GpgTest < Homebrew::TestCase
+  def setup
+    skip "GPG Unavailable" unless Gpg.available?
+    @dir = Pathname.new(mktmpdir)
+  end
+
+  def teardown
+    @dir.rmtree
+  end
+
+  def test_create_test_key
+    Dir.chdir(@dir) do
+      with_environment("HOME" => @dir) do
+        shutup { Gpg.create_test_key(@dir) }
+        assert_predicate @dir/".gnupg/secring.gpg", :exist?
+      end
+    end
+  end
+end