fix(tests): Make .mocharc.js CWD-independent (#7329)

Move tests/browser/test/.mocharc.js to tests/browser and use
__dirname to make the require directive work regardless of where
mocha is invoked from.

Simplify the browser:test script accordingly, taking advantage
also of the mocha default of running all tests in the test/
subdirectory.

tests/browser/.mocharc.js

@@ -2,5 +2,5 @@
 
 module.exports = {
   ui: 'tdd',
-  require: 'tests/browser/test/hooks.js',
+  require: __dirname + '/test/hooks.js',
 };