add checks for LB URIs in config

Author: Divjot Arora

.evergreen/config.yml

@@ -533,6 +533,17 @@ functions:
               fi
           fi
 
+          # Verify that the required LB URI env vars are set to ensure that the test runner can correctly connect to
+          # the LBs.
+          if [ ! -z ${SINGLE_MONGOS_LB_URI} ]; then
+            echo "SINGLE_MONGOS_LB_URI" must be set for testing against LBs"
+            exit 1
+          fi
+          if [ ! -z ${MULTI_MONGOS_LB_URI} ]; then
+            echo "MULTI_MONGOS_LB_URI must be set for testing against LBs"
+            exit 1
+          fi
+
           # Per the LB testing spec, the URI of an LB fronting a single mongos should be used to configure internal
           # testing Client instances, so we set MONGODB_URI to SINGLE_MONGOS_LB_URI.