test(e2e): wait for theme-directives.html

The `theme-directives.html` test currently sometimes takes a screenshot
before all of the Mermaid diagrams have completed rendering.

cypress/integration/rendering/conf-and-directives.spec.js

@@ -207,6 +207,16 @@ graph TD
     it('diagrams should not taint later diagrams', () => {
       const url = 'http://localhost:9000/theme-directives.html';
       cy.visit(url);
+
+      const diagrams = 6;
+      cy.get('svg').should(($svg) => {
+        expect($svg).to.have.length(diagrams);
+        $svg.each((_index, svg) => {
+          expect(cy.$$(svg)).to.be.visible;
+        });
+      });
+      // confirm that we are actually expecting 6 diagrams
+      cy.get('h1').should('have.length', diagrams);
       cy.matchImageSnapshot('conf-and-directives.spec-when-rendering-several-diagrams-diagram-1');
     });
   });