Enable parallelism in test projects

[rbev]

This pull request introduces parallelization settings for NUnit test projects and marks specific acceptance test classes as non-parallelizable to avoid concurrency issues. The main changes are grouped into test project configuration updates and test class attribute adjustments.

Test Project Configuration:

• Added assembly-level NUnit attributes to NUnitSettings.cs in both src/Particular.LicensingComponent.UnitTests and src/ServiceControl.AcceptanceTests.RavenDB to enable parallel test execution with ParallelScope.All and a maximum level of parallelism set to 4.

Test Class Attribute Adjustments:

• Marked all CORS-related acceptance test classes in src/ServiceControl.AcceptanceTests/Security/Cors as [NonParallelizable] to prevent parallel execution and potential test interference.
• Marked all Forwarded Headers-related acceptance test classes in src/ServiceControl.AcceptanceTests/Security/ForwardedHeaders as [NonParallelizable] for test isolation.
• Marked the HTTPS test class When_hsts_is_configured_in_development_mode as [NonParallelizable].

[johnsimons]

Should we just use the default for LevelOfParallelism https://docs.nunit.org/articles/nunit/writing-tests/attributes/levelofparallelism.html, which mean not using the attribute?

[rbev]

Should we just use the default for LevelOfParallelism https://docs.nunit.org/articles/nunit/writing-tests/attributes/levelofparallelism.html, which mean not using the attribute?

I removed the explicit setting and it ran much slower, the build agents don't have 4 cores and benefit from having higher parallelism.