VS Code: prevent installed extensions from breaking tests

On my machine, some Visual Studio Code extension I have installed is
causing tests to hang. Disable installed extensions to avoid potential
conflicts, fixing the hangs on my my machine.

I think this commit should have no effect for fresh VS Code
installations.

Author: strager

plugin/vscode/test/run-vscode-tests.js

@@ -19,7 +19,7 @@ async function mainAsync() {
     extensionDevelopmentPath: path.resolve(__dirname, ".."),
     extensionTestsPath: path.resolve(__dirname, "vscode-tests.js"),
     version: "1.49.0",
-    launchArgs: [workspacePath],
+    launchArgs: ["--disable-extensions", workspacePath],
     extensionTestsEnv: {
       [testFilterEnvironmentVariable]: process.argv[2],
       // HACK(strager): Silence lots of log noise generated by VS Code.