OK,
1. you created the backup of your present site and moved to the test subdirectory.
2.Copied the original sql database in PhpMyAdmin to something like "test1"
3. Went back into the test root directory and modified the conf_global.php where the line for sql database points to the new database "test1" While assuming this to be on the same server the sql user and password are the same.
4. Changed the board URL to be pointed to the new location "http://forum.com/test"
5. Go into the ACP - System - System settings. Go to General Configuration - Find "Path to 'upload' directory" below text box you will see the recommended uploads path copy and paste this into the text box. Do the same for "Upload URL" and Save. Do the same under the IPS Apps if you have the blog and gallery.
6. Go to Look and Feel - Template Tools (In Left sidebar) - Rebuild Master Skin Data check all boxes including the 3 on the left. Hit Rebuild.
7. (Above) Select a skin set - <All Skin Sets> Hit Recache Skin Sets.
8. Go back to System - Cache Management - Rebuild Furl Cache - Rebuild Global Caches Cache - Recache All.
9. Go to Applications & Modules - Manage Applications & Modules - Hit Recache Apps & Modules.
10. Go to Applications & Modules - Manage Hooks - Hit Reimport Applications Hooks.
No go to front end and refresh.... Do you have your test site?
If this is still not working, what you may want to try (if you still have a copy of 3.4.6) is save these directories Cache, downloads (if you have it), hooks public, screenshots (if you have it), and Uploads. Also save the conf_global.php from above. Via FTP upload the fresh copy of 3.4.6 to this test site. If you do not have a copy of 3.4.6 then you will need to upgrade the live board to 3.4.8 and start this whole process over starting from the beginning.
Good Luck and Merry Christmas!