Do not reregister watchers on every file change for qltests

[koesie10]

During the qltest discovery, we were recreating the watchers for qltests on every file change. This was causing the watchers to be recreated on each change, while there were no functional changes to the watchers themselves.

This commit moves the creation of the watchers to the constructor of the QLTestDiscovery class, and removes the creation of the watchers from the discover() method. The behavior should be the same.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[robertbrignull]

I also can't see any reason to recreate the watchers each time. As far as I can tell there isn't anything that could change between calls that would mean we need to remake them. The this.workspaceFolder is readonly and that's the only input.

cc @dbartol just in case he can see something / remembers something.

However, I think go ahead merging this when you're happy, and we can easily merge more changes to this area if we discover this has changed behaviour.