Concurrency error with reader registry

[darrelmiller]

Describe the bug
Error when running tests. We need to make the registry of readers a concurrent directory.

System.InvalidOperationException : Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
at System.Collections.Generic.Dictionary2.TryInsert(TKey key, TValue value, InsertionBehavior behavior) at System.Collections.Generic.Dictionary2.set_Item(TKey key, TValue value)
at Microsoft.OpenApi.Reader.OpenApiReaderRegistry.RegisterReader(String format, IOpenApiReader reader) in

OpenApi File To Reproduce
Run the unit tests

Expected behavior
All tests to pass

Screenshots/Code Snippets

OpenAPI.NET/src/Microsoft.OpenApi/Reader/OpenApiReaderRegistry.cs

Line 29 in efa812a

_readers[format] = reader;

Additional context
Add any other context about the problem here.