Add test for reading multiple Kustomizations in a single file

Signed-off-by: Marcel Boehm <marcel.boehm@inovex.de>

internal/build/testdata/local-kustomization/multi-doc-reset.yaml

@@ -0,0 +1,18 @@
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: first
+  namespace: flux-system
+spec:
+  path: "./k8s/first"
+  components:
+  - foo
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: podinfo
+  namespace: flux-system
+spec:
+  path: "./k8s/second"
+---