Fix gem generation and smoke test run.

[fudanchii]

The newer version of bundle (e.g. 2.1.4) seems to have new option to automatically include either rspec or minitest, e.g. via:

BUNDLE_GEM__TEST: "rspec"

Check existing gem 'rspec' declaration and skip Gemfile patching if it's exist.

Also fix smoke test to patch the spec first so we can have success rspec run again.