Test: dreport: Add entity-manager's system.json to dump

Add /var/configuration/system.json to BMC dumps as em-system.json.  This
file contains a JSON representation of the objects that entity-manager
puts on D-Bus and can be used to debug problems when entity-manager
crashes.

Tested:

~/dumps/BMCDUMP.13BE960.00000000.20220802172737_out/archive$ ls -l em-system.json
-rw-r--r--. 1 spinler spinler 10688 Aug  2 12:27 em-system.json

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: Icc4e38e9594b578b83b65a23af8da67e514572c2