Test Plan for Partial Events and Constructors

[jjonescz]

Champion issue: dotnet/csharplang#9058
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/partial-events-and-constructors.md
Feature branch: https://github.com/dotnet/roslyn/tree/features/PartialEventsCtors

• language version (LangVersion)
  • partial event
  • constructor: partial treated as type previously
• partial before other modifiers (Partial*Last)
• in non-partial class or struct (NotInPartialType)
• in interface
• missing implementation (MissingImplementation)
• missing declaration (MissingDefinition)
• duplicate implementation (DuplicateImplementation)
• duplicate declaration (DuplicateDefinition)
• static constructor (StaticPartialConstructor)
• extern (Extern_*)
• abstract (Abstract)
• explicit interface implementation event (ExplicitInterfaceImplementation)
• event with initializer (EventInitializer_*)
• missing accessors (EventAccessorMissing)
• merged attributes
• mismatched static and instance modifiers
• mismatched accessibility
• use partial event as value (Use_EventAsValue)