test: Add support for out-of-band storage of test/reference

test/browser/main.fmf

@@ -10,5 +10,6 @@ require:
   - make
   - npm
   - python3
+  - python3-pillow
 test: ./browser.sh
 duration: 60m

test/browser/run-test.sh

@@ -4,7 +4,7 @@ set -eux
 # tests need cockpit's bots/ libraries and test infrastructure
 cd $SOURCE
 git init
-make bots test/common
+make bots test/common test/reference
 
 # only install a subset to save time/space
 rm -f package-lock.json  # otherwise the command below installs *everything*, argh