VSCode - Debugging / Testing - TextEditor

I implemented a vscode extension and now I want to test it. My extension consists of 3 functions, I was able test 2 of them using vscode-test's runTest and mocha and this launch config:

{
    "name": "Extension Test",
    "type": "extensionHost",
    "request": "launch",
    "runtimeExecutable": "${execPath}",
    "args": [
        "--extensionDevelopmentPath=${workspaceFolder}",
        "--extensionTestsPath=${workspaceFolder}/out/extension/tests/suite/index"
    ],
    "outFiles": [
        "${workspaceFolder}/out/extension/tests/**/*.js"
    ]
}

But I am stuck now, because the third function needs a vscode.TextEditor and I can't find a way to create or get one. Launching this doesn't give the test files any TextEditors, I tried to get one using:

let editor1 = vscode.window.visibleTextEditors; // --> length = 0
let editor2 = vscode.window.activeTextEditor; // --> undefined

Is there a way to test this?