automake: set NM before running the tests

Set/export the NM variable since it may not be set already.

Fixes: 4f08bfe9 ("*-symbol-check: Don't hard-code nm executable")
Cc: Heiko Becker <heirecka@exherbo.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>