x/tools/gopls: add extra testdata in gopls module

[stamblerre]

CL 209220 doesn't work because the diff algorithm in golang.org/x/tools/internal/lsp doesn't support sub-line diffs. We should probably have a way to add extra testdata in the golang.org/x/tools/gopls so that we can add extra tests that only work inside of that module.

/cc @ianthehat @heschik

[gopherbot]

Change https://golang.org/cl/214280 mentions this issue: internal/lsp: fix link handling for links without schemes