🐛 Increase timeout to 1.5x the asset discovery timeout (30s) (#380)

Assets that take a long time to respond in asset discovery can cause the
`cy.then` promise to timeout. We _must_ wait for all assets to be
discovered/resolved/captured before moving onto the next test, or we risk
capturing the wrong asset for that given snapshot. For example, if the server
shuts down after each test, we would fail to capture those assets if we didn't
await on asset discovery to finish.

With that said, we can safely increase this timeout beyond asset discoveries
timeout since asset discovery will timeout first.

index.js

@@ -4,6 +4,9 @@ const utils = require('@percy/sdk-utils');
 const sdkPkg = require('./package.json');
 const CLIENT_INFO = `${sdkPkg.name}/${sdkPkg.version}`;
 const ENV_INFO = `cypress/${Cypress.version}`;
+// asset discovery should timeout before this
+// 1.5 times the 30 second nav timeout
+const CY_TIMEOUT = 30 * 1000 * 1.5;
 
 // Maybe set the CLI API address from the environment
 utils.percy.address = Cypress.env('PERCY_SERVER_ADDRESS');
@@ -31,7 +34,7 @@ Cypress.Commands.add('percySnapshot', (name, options) => {
   // Default name to test title
   name = name || cy.state('runnable').fullTitle();
 
-  cy.then(async () => {
+  return cy.then({ timeout: CY_TIMEOUT }, async () => {
     if (Cypress.config('isInteractive') &&
         !Cypress.config('enablePercyInteractiveMode')) {
       return cylog('Disabled in interactive mode', {
@@ -52,7 +55,7 @@ Cypress.Commands.add('percySnapshot', (name, options) => {
     }
 
     // Serialize and capture the DOM
-    return cy.document({ log: false }).then(dom => {
+    return cy.document({ log: false }).then({ timeout: CY_TIMEOUT }, dom => {
       let domSnapshot = window.PercyDOM.serialize({ ...options, dom });
 
       // Post the DOM snapshot to Percy