Fix Puppeteer instance teardown (#1586)

The code was not waiting for the browser instance to get properly closed. That
did not affect the result of the crawl, but the OS could report an error
message such as "A child process was not properly killed" when the crawl
command exited.

src/lib/specs-crawler.js

@@ -504,7 +504,7 @@ async function crawlList(speclist, crawlOptions) {
 
     // Close Puppeteer instance
     if (!crawlOptions.useCrawl) {
-        teardownBrowser();
+        await teardownBrowser();
     }
 
     return results;