Change link test task to trigger both lambda

This modifies the previous "(_)send_link_test" method to trigger a
link test for a specific lambda. We then call the method with both
the primary and failover lambda in new orchestrator method.

Since the _invoke_lambda function doesn't raise exceptions if it
fails, there's no need to rescue anything in order to ensure the
second link test / invocation runs as well. It doesn't testing for
this, since it boils to an absence of code to raise any exception.

Note that, like the other parent tests, we only check the new method
works with a specific proxy client instance.

app/clients/cbc_proxy.py

@@ -78,11 +78,14 @@ class CBCProxyClientBase(ABC):
     def __init__(self, lambda_client):
         self._lambda_client = lambda_client
 
-    def send_link_tests(self):
+    def send_link_test(self):
         self._send_link_test(self.lambda_name)
         self._send_link_test(self.failover_lambda_name)
 
-    def _send_link_test(self): pass
+    def _send_link_test(
+        self,
+        lambda_name,
+    ): pass
 
     def create_and_send_broadcast(
         self, identifier, headline, description, areas, sent, expires, channel, message_number=None