refactor: Adopt to the new VS Code tests API

[jdneo]

address #1162

[mfussenegger]

I think this might have caused a regression. Retrieving the junit arguments now takes a lot longer:

[2021-08-03T20:21:21+0200] "vscode.java.test.junit.argument", "start"
[2021-08-03T20:22:07+0200] "vscode.java.test.junit.argument", "finished"

That's almost 50 seconds.

Compared to the latest version on the master branch (instead of main):

[2021-08-03T20:25:55+0200] "vscode.java.test.junit.argument", "start"
[2021-08-03T20:25:55+0200] "vscode.java.test.junit.argument", "finished"

That only took a couple of ms.

I took a profile from the main branch:

And one from before:

I tested this with nvim-jdtls (with mfussenegger/nvim-jdtls#108) on the CrateDB code base. (Within the PlannerTest class)