Make sure map loaded in test before testing layers

Have seen a situation (more likely with upgraded jQuery?) where
the test code runs before the map has initialised its layers.

.cypress/cypress/integration/bathnes.js

@@ -16,6 +16,7 @@ it('loads the staff layer correctly', function() {
     cy.get('[name=pc]').parents('form').submit();
     cy.url().should('include', '/around');
     cy.window().its('fixmystreet.maps').should('have.property', 'banes_defaults');
+    cy.get('.olMap'); // Make sure map is loaded before testing its layers
     cy.window().then(function(win){
         var llpg = 0;
         win.fixmystreet.map.layers.forEach(function(lyr) {