WorkItem Attribute needs to be added in.

[AbhitejJohn]

Description

WorkItemAttribute was in V1 and migrating to V2 causes build failures. This needs to go via an extensibility point though.

Steps to reproduce

1. Create a MSTest V1 project with a WorkItemAttribute.
2. Migrate to MSTest V2.
3. Build project

Expected behavior

Project build works.

Actual behavior

Build fails with a cannot find a WorkItemAttribute.

Environment

This should repro across.

[AbhitejJohn]

We've added all the Description attributes that were present in MSTest V1 into V2 following the extensibility defined here: https://github.com/Microsoft/testfx/blob/master/docs/001-Framework-Extensibility-Description-Attributes.md .