add shadow.loader.TEST define
add shadow.loader.TEST define

allows using shadow.loader in karma targets by actually ignoring
modules and just assuming code is already loaded manually.

fixes #889

thheller/shadow-cljs,thheller/shadow-devtools,thheller/shadow-devtools,thheller/shadow-cljs,thheller/shadow-devtools,thheller/shadow-cljs,thheller/shadow-cljs,thheller/shadow-devtools