Cleaned up duplication in push command tests

The push command tests were testing the Gem::GemcutterUtilities#sign_in
instead of any command functionality.  The --host option was not tested
(despite that being the intent of the test).

The duplicate tests were deleted and replaced by a single test for
Gem::Commands::PushCommand#send_gem.

test_execute_host was added to test the --host option.

Discovered by hltbra