Copying request params before Core.php exec in PHPUnit bootstrap

Same behaviour as with 3.0 bootstrap.php and the
2.4 cli-script.php (which it is based on).
This allows to use GET switches which are evaluated in _config.php
files, e.g. db=<db-alias> settings for running tests
with various DBs without changing the underlying PHP config.
Update documentation of nginx configuration

- Avoid using "if" to check for file existence (use try_files instead)
- Replicate the behavior of the .htaccess files
- TODO: get static error pages to work