core(config): use abbreviation for pwa category title

lighthouse-core/config/default-config.js

@@ -96,7 +96,7 @@ const UIStrings = {
   /** Description of the navigation section within the Search Engine Optimization (SEO) category. Within this section are audits with descriptive titles that highlight ways to make a website accessible to search engine crawlers. */
   seoCrawlingGroupDescription: 'To appear in search results, crawlers need access to your app.',
   /** Title of the Progressive Web Application (PWA) category of audits. This is displayed at the top of a list of audits focused on topics related to whether or not a site is a progressive web app, e.g. responds offline, uses a service worker, is on https, etc. Also used as a label of a score gauge. */
-  pwaCategoryTitle: 'Progressive Web App',
+  pwaCategoryTitle: 'PWA',
   /** Description of the Progressive Web Application (PWA) category. This is displayed at the top of a list of audits focused on topics related to whether or not a site is a progressive web app, e.g. responds offline, uses a service worker, is on https, etc. No character length limits. 'Learn More' becomes link text to additional documentation. */
   pwaCategoryDescription: 'These checks validate the aspects of a Progressive Web App. ' +
   '[Learn more](https://developers.google.com/web/progressive-web-apps/checklist).',

lighthouse-core/test/results/sample_v2.json

@@ -6655,7 +6655,7 @@
       "score": 0.67
     },
     "pwa": {
-      "title": "Progressive Web App",
+      "title": "PWA",
       "description": "These checks validate the aspects of a Progressive Web App. [Learn more](https://developers.google.com/web/progressive-web-apps/checklist).",
       "manualDescription": "These checks are required by the baseline [PWA Checklist](https://developers.google.com/web/progressive-web-apps/checklist) but are not automatically checked by Lighthouse. They do not affect your score but it's important that you verify them manually.",
       "supportedModes": [