tests: reset _found at the end of each iteration (#8195)

This makes `inst_rules` really print all rules it skips.

test/test-functions

@@ -1058,6 +1058,7 @@ inst_rules() {
             fi
         done
         [[ $_found ]] || dinfo "Skipping udev rule: $_rule"
+        _found=
     done
 }