If the WordPress install is using an SVN checkout of trunk, the developer plugin should detect that, reflect it in the UI, and not prompt to install the beta tester plugin
if ( is_dir( ABSPATH . '/.svn/' ) )
If we have an svn folder we aren't sure it's trunk, we just know it's checked out from svn.
First, we could try to run svn info in the ABSPATH. If this doesn't work, we can read the .svn/entries file and check if the first line starting with http:// is the URL to WordPress trunk.