VSCode - Typescript - Debug / Test

I wrote an extension, and I want to test it. One function needs a vscode.TextEditor. Using 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"
    ]
}

I am using vscode-test.runTests and mocha. In my test file I try to create a TextEditor, by doing:

let file: vscode.Uri;
file = vscode.Uri.parse('file:' + __dirname + '/test.txt');
vscode.workspace.openTextDocument(file).then(doc => {
    vscode.window.showTextDocument(doc).then(editor => {
        console.log(editor.document.getText);
    });
}, () => {
    console.log('something went wrong')
});

the string I used in the parse method of vscode.Uri is a Uri of a test.txt file I created with some test data.

The problem is, that the openTextDocument always gets to the onrejected function ('something went wrong').

Am doing something wrong? Why is that the case?