Review tests, add additional documentation to emphasize intentions

[atc0005]

Currently I find the tests to be both to be brittle and not as clear as they could be. This is probably from a lack of an "ah ha!" moment.

I've made attempts to refactor with recent releases, but further work is needed to document exactly what is happening and why.

For example, one thing I spent an embarrassing amount of time on this AM was "remembering" the relationship between resError and error in the table test entries:

go-teams-notify/send_test.go

Lines 31 to 39 in cea9269

	var tests = []struct {
	reqURL string
	reqMsg MessageCard
	resStatus int // httpClient response status
	resBody string // httpClient response body text
	resError error // httpClient error
	error error // method error
	skipURLVal bool // whether webhook URL validation is applied (e.g., GH-68)
	}{

If I recall correctly, you're specifying what client error is expected (e.g., connectivity issue) and what error is expected from invoking library functions/methods using provided table test data.

This table test entry is an example of where there is no client error, but there is an error returned from validating the target endpoint or request URL:

go-teams-notify/send_test.go

Lines 50 to 59 in cea9269

	// invalid webhookURL - missing prefix in webhook URL
	{
	reqURL: "",
	reqMsg: simpleMsgCard,
	resStatus: 0,
	resBody: "invalid",
	resError: nil,
	error: ErrWebhookURLUnexpectedPrefix,
	skipURLVal: false,
	},