Bug 1139998 - Make it more clear in docs how to set things up to run UI tests

docs/ui/installation.rst

@@ -47,10 +47,15 @@ Requirements
 ------------
 
 * Node.js_
-* karma: ``sudo npm install -g karma``
 
 Execution::
 
+First run ``npm install`` at the root to install local dependencies
+(if you haven't already done so). To avoid a bunch of path mangling,
+you also want to install karma-cli globally (``sudo npm install -g karma-cli``).
+
+Once that's done, you can just run this shell script to execute the tests:
+
     ./tests/ui/scripts/test.sh
 
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