Duplicate subscriptions for notifications
# Issue
Currently 2 tests are present to address the situations labelled `DUPLICATION` and `NOT-DUPLICATION`. Only one of the tests is run according to the value of the `VNF_DUPLICATION` variable.
This may lead to confusion.
# Proposal
* [x] Rename the variable to a more meaningful name: `VNFM_ALLOWS_DUPLICATE_SUBS`
* [x] Merge the tests into one test (for each API) where an `If` keyword selects response code and body
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