Add missing tests

[idg10]

Test coverage currently stands at about 33%, which is symptomatic of the fact that significant chunks of this code either don't have unit tests, or have at best patchy tests. We need to look at whether we need to add tests on:

• Auditing
• Converters
• Exception handling and mapping
• Certain HAL aspects (e.g., TryGetProperties, links)
• Operation locator
• Configuration handling
• Context building
• External URL resolution
• OpenApiServiceOperation with parameters
• Link resolution (failure cases, and any cases for anything other than by-owner-and-relation)
• Validation
• Operation path template handling
• Service definition handling
• Hosting