Stop using ; as a separator in URL query strings

Go 1.17 (by default) disallows using ; as a separator. Our tests fail
on Go 1.17 with the following error. Since we only do this in test
code, it's easiest just to switch to & so we don't have a breakage
when we start compiling with 1.17.

See also: golang/go#25192

Error:

    2021/09/03 10:21:57 http: URL query contains semicolon, which is no longer a supported separator; parts of the query may be stripped when parsed; see golang.org/issue/25192

    ----------------------------------------------------------------------
    FAIL: apiserver_test.go:473: WebSuite.TestSAMLSuccess

    apiserver_test.go:524:
        c.Assert(u.Scheme+"://"+u.Host+u.Path, Equals, fixtures.SAMLOktaSSO)
    ... obtained string = ":///web/msg/error/login"
    ... expected string = "https://dev-813354.oktapreview.com/app/gravitationaldev813354_teleportsaml_1/exkafftca6RqPVgyZ0h7/sso/saml"

lib/web/apiserver_test.go

@@ -498,7 +498,7 @@ func (s *WebSuite) TestSAMLSuccess(c *C) {
 
 	csrfToken := "2ebcb768d0090ea4368e42880c970b61865c326172a4a2343b645cf5d7f20992"
 
-	baseURL, err := url.Parse(clt.Endpoint("webapi", "saml", "sso") + `?redirect_url=http://localhost/after;connector_id=` + connector.GetName())
+	baseURL, err := url.Parse(clt.Endpoint("webapi", "saml", "sso") + `?redirect_url=http://localhost/after&connector_id=` + connector.GetName())
 	c.Assert(err, IsNil)
 	req, err := http.NewRequest("GET", baseURL.String(), nil)
 	c.Assert(err, IsNil)