Investigate testing DAP features

[tsmaeder]

Investigate writing automated tests as requested in #13897

The goal of this issue is to come up with a way to write tests for VS Code extensions that provide debug adapters.

We'd have to be able to script

1. Setting up a workspace
2. Creating debug configurations and launching debuggees (and connecting to them)
3. Exceute debug protocol actions
4. Intercepting and verifying results of those debug actions

I would prefer to write the scripts based on a API, not based on UI interactions

[tolusha]

It is possible to initialize and control debug session using Theia frontend API [1]
Here is the testing workspace [2]

[1] https://github.com/theia-ide/theia/blob/ab/api-tests/packages/apt-tests/src/browser/debug-tests.ts
[2] https://github.com/theia-ide/theia/tree/ab/api-tests/packages/apt-tests/testWorkspace

The next step is to create a test suite like VS Code does [3] and run it using launch configuration [4]

[3] https://github.com/microsoft/vscode/tree/master/extensions/vscode-api-tests
[4] eclipse-theia/theia#4224 (comment)

[tolusha]

The work is done here https://github.com/theia-ide/theia/tree/ab/theia-tests/packages/theia-tests

Workflow is the following:

1. Start Theia
2. Open Theia project
3. Run the configuration below

{
      "name": "Launch Theia Tests",
      "type": "node",
      "request": "launch",
      "args": [
        "${workspaceFolder}/examples/browser/src-gen/backend/main.js",
        "${workspaceFolder}/packages/theia-tests/testWorkspace",
        "--port",
        "3030",
        "--hostname",
        "0.0.0.0"
      ],
      "env": {
        "THEIA_DEFAULT_PLUGINS": "local-dir:${workspaceFolder}/plugins"
      },
      "stopOnEntry": false,
      "sourceMaps": true,
      "outFiles": [
        "${workspaceFolder}/../.js"
      ]
    }

4. Opens browser:
   http://localhost:3030/?runTests
?runTests is a crucial thing which tells TestRunner to start tests.

5. Wait until tests are finished

root INFO -------------------------------------------------------------
root INFO Results:
root INFO Test run: 2 Passed: 2 Failures: 0
root INFO -------------------------------------------------------------

Implementation details.

Tests are run when test environment is set [1]. TestRunner [2] scans test classes [3] for test methods and run them one by one.

[1] https://github.com/theia-ide/theia/blob/ab/theia-tests/packages/theia-tests/src/browser/theia-tests-frontend-module.ts
[2] https://github.com/theia-ide/theia/blob/ab/theia-tests/packages/theia-tests/src/browser/framework/test-runner.ts
[3] https://github.com/theia-ide/theia/blob/ab/theia-tests/packages/theia-tests/src/browser/framework/test-case.ts

TODO

1. Move theia-test extension to https://github.com/eclipse/che-theia/tree/master/extensions
2. Run tests inside Eclipse Che