GODT-1159 SMTP server not restarting after restored internet

- [x] write tests to check that IMAP and SMTP servers are closed when there
  is no internet
- [x] always create new go-smtp instance during listenAndServe(int)

test/mocks/imap_response.go

@@ -117,7 +117,7 @@ func (ir *IMAPResponse) AssertResult(wantResult string) *IMAPResponse {
 func (ir *IMAPResponse) AssertError(wantErrMsg string) *IMAPResponse {
 	ir.wait()
 	if ir.err == nil {
-		a.Fail(ir.t, "Expected error %s", wantErrMsg)
+		a.Fail(ir.t, "Error is nil", "Expected to have %q", wantErrMsg)
 	} else {
 		a.Regexp(ir.t, wantErrMsg, ir.err.Error(), "Expected error %s but got %s", wantErrMsg, ir.err)
 	}