Provide an extension directory when running UI tests

[xorye]

Since vscode-extension-tester version 2.5: https://github.com/redhat-developer/vscode-extension-tester/blob/master/CHANGELOG.md#250-march-20, it was made possible to explicitly provide a directory where extensions are to be installed for the test instance of VS Code.

Currently, the test instance of VS Code detects and installs extensions in the ~/.vscode/extensions directory, which is the default extensions directory for VS Code.

It would be better to install the test instance's extensions somewhere else, so that it doesn't conflict with already-installed extensions located in ~/.vscode/extensions.