Add should package

[kegsay]

The should package is a more lenient form of the must package, in that it does not fail the test if the assertion fails. Instead, these functions return an error, which the test can then handle.

This is particularly useful in cases where a test needs to retry a certain request. Previously, you had to hand-roll the checks on the response because using must.MatchResponse would fail the test, even though it might have been okay as the next retry may pass the matchers.

Fixes #546

Tests need to be revisited to see if this can be used in more places.

An example of this. Before:

res = alice.DoFunc(
	t,
	"GET",
	[]string{"_matrix", "client", "v3", "rooms", roomID, "aliases"},
	client.WithRetryUntil(
		1*time.Second,
		func(res *http.Response) bool {
			if res.StatusCode != 200 {
				return false
			}
			eventResBody := client.ParseJSON(t, res)
			matcher := match.JSONKeyEqual("aliases", []interface{}{roomAlias})
			err := matcher(eventResBody)
			if err != nil {
				t.Log(err)
				return false
			}
			return true
		},
	),
)

After:

res = alice.Do(
	t,
	"GET",
	[]string{"_matrix", "client", "v3", "rooms", roomID, "aliases"},
	client.WithRetryUntil(
		1*time.Second,
		func(res *http.Response) bool {
			_, err := should.MatchResponse(res, match.HTTPResponse{
				StatusCode: 200,
				JSON: []match.JSON{
					match.JSONKeyEqual("aliases", []interface{}{roomAlias}),
				},
			})
			if err != nil {
				t.Log(err)
				return false
			}
			return true
		},
	),
)

[DMRobertson]

Seems very sensible.