x/tools/gopls: add regression tests for file watching #40818
Description
Some cases that lack coverage:
- Moving a declaration to a new file
- Adding a new declaration to the same file
- Adding a new declaration a different file
- Check that closing a buffer clears errors
- Real-life example: You switch branches with 1 open file in your editor and see errors because that file is now an overlay. You close the file and its errors clear.