unit tests

[yegor256]

Let's add unit tests, using Ruby webmock. At the moment the code is not tested properly.

[0crat]

@yegor256/z I'm not managing this repo, remove the webhook or contact me in Slack, as explained in §11; I'm not managing zold-io/zold-ruby-sdk GitHub repository, you have to contact me in Slack first, see §11 /cc @yegor256/z

[edmoffo]

The repository carries test/zold/test_wts.rb and webmock is already in the Gemfile, so the scaffolding the original report asked for is in place; the test contract is still incomplete though. Six of the eight tests open with WebMock.allow_net_connect! and hit https://wts.zold.io directly, so CI depends on the live WTS service rather than on stubs. Only test_works_with_webmock actually stubs the HTTP call. Closing this issue requires migrating each WebMock.allow_net_connect! test to a stub against the URL lib/zold/wts.rb calls for pull, wait, id, balance, find, pay, and usd_rate, then enabling WebMock.disable_net_connect! in test/test__helper.rb so a network call from any future test fails loudly.