tests: For installed, s/test-/itest-/ to avoid in-tree name clashes

I want to migrate `test-pull-many.sh` → `itest-pull.sh`, hence not
conflicting with the unit test `test-pull.sh.

Closes: #840
Approved by: jlebon

tests/installed/run.sh

@@ -3,7 +3,7 @@
 set -xeuo pipefail
 
 dn=$(dirname $0)
-for tn in ${dn}/test-*.sh; do
+for tn in ${dn}/itest-*.sh; do
     echo Executing: ${tn}
     ${tn}
 done