use Eventually in RunURLAssertSuite test

The RunURLAssertSuite harness currently waits on finalizers
added to MariaDBAccount as an indication when it's safe to
test that the new URL is present in the config.  however,
in [1] we are discussing removing the use of finalizers here so
that this harness can be generalized to more kinds of controllers.
as such, if the assertion here uses Eventually, it becomes compatible
with this change, as the waiting for the condition is implemented
directly here.

[1]
openstack-k8s-operators/mariadb-operator#205 (review)

tests/functional/placementapi_controller_test.go

@@ -919,13 +919,15 @@ var _ = Describe("PlacementAPI controller", func() {
 	mariadb_suite.RunBasicSuite()
 
 	mariadb_suite.RunURLAssertSuite(func(accountName types.NamespacedName, username string, password string) {
-		cm := th.GetSecret(names.ConfigMapName)
+		Eventually(func(g Gomega) {
+			cm := th.GetSecret(names.ConfigMapName)
 
-		conf := cm.Data["placement.conf"]
+			conf := cm.Data["placement.conf"]
 
-		Expect(string(conf)).Should(
-			ContainSubstring(fmt.Sprintf("connection = mysql+pymysql://%s:%s@hostname-for-openstack.%s.svc/placement?read_default_file=/etc/my.cnf",
-				username, password, namespace)))
+			g.Expect(string(conf)).Should(
+				ContainSubstring(fmt.Sprintf("connection = mysql+pymysql://%s:%s@hostname-for-openstack.%s.svc/placement?read_default_file=/etc/my.cnf",
+					username, password, namespace)))
+		}, timeout, interval).Should(Succeed())
 
 	})