NIAD-2851: Prevent duplicate 'Laboratory' categories when mapping SpecimenCompounds

[MartinWheelerMT]

What

Prevent an observation from containing duplicated "Laboratory" categories.

Why

If an Observation being handled by the SpecimenCompoundsMapper already contained a `"Laboratory" category then it would add a new one, resulting in duplication, such as the example below:

{
            "resource": {
                "resourceType": "Observation",
                "id": "C515E71E-2473-11EE-808B-AC162D1F16F0",
                "meta": {
                    "profile": [
                        "https://fhir.nhs.uk/STU3/StructureDefinition/CareConnect-GPC-Observation-1"
                    ]
                },
                "identifier": [
                    {
                        "system": "https://PSSAdaptor/B83002",
                        "value": "C515E71E-2473-11EE-808B-AC162D1F16F0"
                    }
                ],
                "status": "final",
                "category": [
                    {
                        "coding": [
                            {
                                "system": "http://hl7.org/fhir/observation-category",
                                "code": "laboratory",
                                "display": "Laboratory"
                            }
                        ]
                    },
                    {
                        "coding": [
                            {
                                "system": "http://hl7.org/fhir/observation-category",
                                "code": "laboratory",
                                "display": "Laboratory"
                            }
                        ]
                    }
                ]...

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users

[adrianclay]

Resolved by #722