tumbler: run the test from the source tree

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3476

common/tumbler-thumbnailing.yaml

@@ -17,7 +17,7 @@ install:
 
 run:
   steps:
-    - common/run-test-in-systemd --timeout=900 --user=user --name=run-test env DEBUG=2 /usr/share/chaiwala-tests/tumbler/automated/run-test.sh
+    - common/run-test-in-systemd --timeout=900 --user=user --name=run-test env DEBUG=2 tumbler/automated/run-test.sh
 
 parse:
   pattern: ^(?P<test_case_id>[a-zA-Z0-9_\-\./]+):\s*(?P<result>pass|fail|skip|unknown)$