test: allow multiple 'has been added' matches in preview test 3

For an undeployed app, the additive diff covers every chart resource, so
the literal 'has been added' header appears once per resource - six times
in the python test app's chart. `assert_output_contains` defaults to
exact-count 1, which fails. Pass -1 (at least 1) to match the actual
semantics of the test.

tests/unit/scheduler-k3s-preview.bats

@@ -87,5 +87,5 @@ teardown() {
   echo "output: $output"
   echo "status: $status"
   assert_success
-  assert_output_contains "has been added"
+  assert_output_contains "has been added" -1
 }