cannot use [cms.root] in admin/setting/path

Setting 'CiviCRM Extensions Directory' to [cms.root]/existing_directory results in the validation error "'CiviCRM Extensions Directory' directory does not exist"

Explanation of problem and solution below...

[cms.root] does not have a trailing slash and is passed CRM_Utils_File::absoluteDirectory as $basePath.

CRM_Utils_File::absoluteDirectory expects the $basePath parameter to have a trailing slash.

Without a trailing slash in $basePath, the absolute directory returns an invalid directory.

For example, for a site with cms.root = /var/www/civicrm "[cms.root]/test" will be resolved to /var/www/civicrmtest (it should be resolved to /var/www/civicrm/test).

Checking to see whether $basePath has a trailing slash and adding one if it doesn't seems like a fairly future proof way to solve this problem in this instance and in others.