Use ":" as separator for the simpletest user agent

Problem/Motivation

Cookies are separated using semicolons, see https://tools.ietf.org/html/rfc2109 | http://curl.haxx.se/rfc/cookie_spec.html
This makes things tricky if something doens't properly encode the values and the value contains a semicolon.

Proposed resolution

Let's make our life easy to don't use semicolon, so we don't have to fix phantomjs in #2469713: Step 2: Create a JavaScriptTestBase using PhantomJs Driver/Binary →

Remaining tasks

User interface changes

API changes

Data model changes