Test failures indicate that this operation doesn't always take effect
immediately:
https://travis-ci.org/hashicorp/terraform/builds/103764466
Add a simple poll to retry a few times until it does.
```
--- PASS: TestAccMailgunDomain_Basic (1.51s)
```
Verified that this does the trick by looping the test and watching the
logs for the retry behavior to kick in.
Fixes the following vet report:
builtin/providers/mailgun/resource_mailgun_domain_test.go:73: arg DomainResp.Domain.Wildcard for printf verb %s of wrong type: bool
Paul Hinze