Support separate Slack channels for notifications of successful and failed test results

[m-kusnierz]

This PR adds support for sending Slack notification to different channels depending on the test results (passed or failed).

Resolves issue: #69

[m-kusnierz]

This condition was not true when this.slackChannels was undefined.
With a change to slightly stricter validation I had to update the test data here 👉 https://github.com/ryanrosello-og/playwright-slack-report/pull/82/files#diff-62ed587fd8e65d151c73c06fa1f2fd6d4e3c45760c124f1505f41fa1d547f8cdR295-R322

[ryanrosello-og]

I tested this just now, works good @m-kusnierz !

And its backwards compatible too (using just plain old channels key)

[RAZAW]

Hey Ryan,
Also in a situation for sending messages to different channels. Just curious where your slackWebhookUrl or other alternatives are in the config?
Secondly, the slack incoming web hooks are specific to a single channel, so how does the channels array work? If you can advise on how to post to multiple channels that would be great.
Just for reference my use cases are:

1. Send message to multiple channels
2. Send message to channel based on test tag

[RAZAW]

Managed to figure out the comment above by using slackoAuthToken property in TestConfig I can use the array of channels. I couldn't see it in your config file above may be you'd have set the env variable at runtime or script. Thanks anway.