browser-tests - disable timeouts for Mocha 1.21

Commit 17a3ca7eb70942a6c74aa50876be1ce31dc1de00 set timeout to Infinity,
but this still doesn't solve the issue reported in #961 if you use Mocha
1.21.

This commit changes the config for grunt test run to
`enableTimeouts=false`, and documents the equivalent command line switch
(`--no-timeouts`) for mocha executable.