Get rid of ugly simpletest regexp.

This implements simpletest_id() and fix two small things:

- the file directory should use the not prefixed id
- a typo

c9606571 noted that the installer was broken and that double prefixing could happen in tearDown().

New version up for review.

Fixed code comments and removed the unused parameter for simpletest_id(), following c9606571 review on IRC.

Some tests fails with this.

Badfully, we will have to make simpletest_id() to work also in the testing site. Back to the drawing board.

@boombatower: I reverted back the file directory to the old behavior. Note anyway that we now add the main database prefix just to make the "clean environment" feature work correctly, not because of potential conflicts (which should not happen because the prefix is random enough).

Also:

- moved the detection of HTTP_USER_AGENT out of the database layer and in conf_init(), where it makes sense.
- cleanup the simpletest test :)

Rerolled without significant changes.

Moved the core of the user agent detection to conf_init(), in order to simplify the logic in simpletest_id, which is now a simple get/set function.

Following chx' review on the IRC:

- renamed inTestingSite() to inChildSite() which is clearer and reworked comments
- remove a redundant new line

As a summary, this patch aims to remove the multiple regexps on HTTP_USER_AGENT or $db_prefix that were all over core, in the benefit of a simpler and clearer simpletest_id() function. It's a building block above which we will be able to implement #195416: Table prefixes should be per database connection, and that will, later, allow us to speed-up simpletest by simply duplicating a pre-installed Drupal instance, instead of rebuilding it at each test run.

Finally, a simpletest_id() function was not enough, so I added a small static DrupalTestRunInfo class.

@c960657: $db_prefix is set at the bottom of conf_init(), after the settings.php file has been included.