changed HashSet to LinkedHashSet to fix flaky test

[mumbler6]

Linked PR / Issue: adobe/aem-core-wcm-components#2894

There is a flaky test in the aem-core-wcm-components project which should be fixed by changing the HashSet to a LinkedHashSet in the collectResourceTags function.

[stefanseifert]

this method is only used by the mock implementation of TagManager#getTagsForSubtree which defines for the return value:

all tags for the given subtree (in no defined order)

so, although it's not expected to have an actual order, it's fine to at least always use the same order to get stable results in unit tests.