test: flatten tests (#2337)

Today, we have tests split into multiple files, with files pulling
tests from some other files.

This patch starts explicitly gathering all tests from the same
`test.js` file.

Drive-By: move one test from `browser.spec.js` into `puppeteer.spec.js`
since it starts browser itself.

test/coverage.spec.js

@@ -176,4 +176,4 @@ module.exports.addTests = function({testRunner, expect}) {
       });
     });
   });
-};
+};