Extension name is hardcoded, leading to failing tests

[andyleejordan]

When the extension name is switched from powershell-preview to just powershell (such as when releasing a stable version like in #3207), the tests fail because of hardcoded constants like const testExtensionId = "ms-vscode.powershell-preview";. We should instead read the current extension ID from the package.json file, which is already done in main.ts.