Cancel/Abort is not honored when triggered before a custom host is launched.

[AbhitejJohn]

Description

When a Cancel/Abort is sent to the DesignModeClient before the custom host is launched(for debugging let's say) we tend to ignore this and continue on with sending out requests to the client to launch the custom host and continue with the workflow.

Steps to reproduce

1. Open a solution with some tests
2. Open Test Explorer
3. Run all tests, quickly cancel before tests are run.
4. From Test menu, Debug All, again quickly cancel in Test Explorer

Expected behavior

The test run should cancel even before the launch custom host is triggered.

Actual behavior

The workflow ignores the cancel/abort operations and continues on.

Environment

latest 15.7 dogfood build of VS.

Repro project: ManualTest_CSharp1.zip

Tracking bug in VS: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/595438