doc: mark bt_test_cb API as not documented

add a comment so the script for checking undocumented defgroups doesn't
report this internal API as not documented

Signed-off-by: David B. Kinder <[email protected]>

Author: dbkinder

doc/api/bluetooth.rst

@@ -18,6 +18,7 @@ BR/EDR (Bluetooth Classic) APIs require :option:`CONFIG_BT_BREDR`.
 .. comment
    not documenting
    .. doxygengroup:: bluetooth
+   .. doxygengroup:: bt_test_cb
 
 Bluetooth Mesh Profile
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