[Cleanup] Decide whether or not to keep ITestProjectIndicator

[duncanp-sonar]

Description

Previously, the old Connected Mode binding filtered out test projects during binding i.e. it would not add settings to a project file if it was a test project. The new unintrusive binding logic doesn't change project files so this logic is no longer called.

The logic for determining whether a project is a test project or not is reasonably complicated: see this Sonar Scanner for .NET wiki page for more information. The SLVS logic should be the same as the Scanner logic.

The test-detection logic is not currently used in SLVS i.e. test code is treated exactly the same as product code, whereas the Sonar scanners do differentiate between test and product code.

If the intention is to make SLVS handle test code differently then the ITestProjectIndicator code would be useful. Otherwise, it can be deleted.

TODO - decide whether to keep the ITestProjectIndicator and implementations.