x/tools/gopls: add regression tests for file watching

[stamblerre]

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.