x/tools/gopls: add regression testing

[stamblerre]

We've been using the govim tests for regression testing, which creates unnecessary load on @myitcv in filing and reproducing issues. It also places too much reliance on one specific editor client. We should develop our own regression tests that use a fake client.

/cc @findleyr

[gopherbot]

Change https://golang.org/cl/217598 mentions this issue: internal/lsp/fake: add fakes for testing editor interaction

[gopherbot]

Change https://golang.org/cl/218278 mentions this issue: internal/lsp/lsprpc: add test for empty diagnostics in deleted files

[gopherbot]

Change https://golang.org/cl/218322 mentions this issue: internal/lsp/lsprpc: add test for definition outside of workspace

[gopherbot]

Change https://golang.org/cl/218698 mentions this issue: internal/lsp/lsprpc: add an LSP forwarder and regtest environment

[gopherbot]

Change https://golang.org/cl/218777 mentions this issue: internal/jsonrpc2/servertest: support both TCP and pipe connection