So a couple of things: first, I would recommend you upgrade from 15.1X49D70.3 immediately on both units, as there a a lot of issues with this code. The current (as of today) JTAC recommended version is 15.1X49D170.
Secondly, in order to copy the configuration over to your test firewall, I would recommend making it a single-member cluster. This way all of your interface references will match the clustered unit and you can keep the two in sync fairly easily (obviously you may need unique IP addresses depending on how connected your test environment is).
On your test SRX Issue the commands:
Now on the test SRX issue
set chassis cluster cluster-id 2 node 0 reboot
This will bring your stand-alone unit up as node 0 of cluster-id 2 (we use 2 here to ensure there are no conflicts with your production unit which is most likely cluster-id 1).
While your test unit is rebooting, log into your production cluster and issue:
show configuration | no-more
Select and copy all this output to a decent Text Editor (Notepad++/Sublime Text)
Log back into your test SRX and issue:
load override terminal
Paste in the configuration from your text editor - depending on how long it is, you may need to do this in sections.
- Hit Enter and then ctrl-d
commit your configuration
Now you have a complete clone of your production environment.
You may see alarms on your stand-alone unit, but this will be because it is "missing" the secondary node and associated interfaces.