test: add a test for file watching API with gopls

[stamblerre]

A file watching integration test would be really useful, since the LSP file watching API is based on VS Code's.

[gopherbot]

Change https://golang.org/cl/244605 mentions this issue: internal/lsp: fix GlobPattern for watching files

[gopherbot]

Change https://golang.org/cl/247684 mentions this issue: internal/lsp: lowercase drive letters on Windows to fix file watching

[gopherbot]

Change https://golang.org/cl/251081 mentions this issue: test/gopls: add integration tests for file watching on-disk

[stamblerre]

Removing this from the vscode-go: gopls by default project because this is not a blocker, though we should have these tests in place before (if ever) gopls changes the file watching glob patterns.