Editors remain open after testing

[tshino]

In the test of this extension, a vscode instance is launched, some text editor tabs are opened on it, and all the tests are performed on that.
The tabs remain open in the next test session thanks to the workspace restoration feature.
As a result, after several runs of the test, we see lots of open tabs on an instance.

I found this slows down the test runs day by day!

We should close all the editors after testing.