Stub vscode 'Tests' API

[alvsan09]

What it does

This enables the possibility to load and start plugins which depend on the 'Tests' API.
The 'Tests' functionality will not be operational but the rest of
the functionality offered by the plugin will be.

How to test

1. Update the supported vscode API version in theia to 1.68.0, at:

dev-packages/application-package/src/api.ts

2. Reproduce the problem in the theia instance before switching to this PR, by installing the following vscode sample: https://github.com/alvsan09/vscode-extension-samples/releases/download/test-provider-prel-0.0.1/test-provider-sample-0.0.1.vsix

• Open a workspace folder containing markdown files (extension .md)
• you shall see the following failure message:
  

2. Update your theia repository to use this PR

• Open the same workspace and make sure the message above is no longer displayed and the '.md' files open fine.

A more realistic test can be achieved using the latest ms-python.python extension, version 2022.14.0.,
however additional API support is needed in addition to this change.
A test branch is provided for this purpose:

1. Update your theia repository to load this PR on top of the branch offering additional missing API as follows:

git fetch
git checkout asl/tests_api_python_test
git cherry-pick asl/test_api_stub
build it and start it.

2. Download the latest python extension via openVSX registry:
   https://open-vsx.org/api/ms-python/python/2022.14.0/file/ms-python.python-2022.14.0.vsix

3. Using the extensions view install the vsix above

4. The extension will attempt to install 4 python related extensions, the ones related to jupyter notes will not be successful however the core python extension will install and be functional, see the following video clip.

tests_api_stub.mp4

The stubbed API has been marked intheia.d.ts with the special tag @stubbed in order to reflect it is as such in the report generated by the eclipse-theia/vscode-theia-comparator

See a portion of the generated report when running it locally:

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[JonasHelming]

Relates to #10669

[JonasHelming]

@alvsan09 Is this ready for a re review?

[alvsan09]

@alvsan09 Is this ready for a re review?
Yes, I had missed requesting it, thanks for catching it !!

[colin-grant-work]

The implementations appear sound for stubs (where applicable) and the plugins activate successfully, and the API is correctly evaluated as either newly supported or newly stubbed by the comparator.

[JonasHelming]

@alvsan09 Let's merge this if possible :-)

[alvsan09]

The commits were squashed and then re-based to latest master just before merging.

[alvsan09]

The implementations appear sound for stubs (where applicable) and the plugins activate successfully, and the API is correctly evaluated as either newly supported or newly stubbed by the comparator.

@colin-grant-work , thanks for the review and testing !!

[alvsan09]

@alvsan09 Let's merge this if possible :-)
@JonasHelming, Done !!