internal/lsp/regtest: add regtest for golang/go#36951
internal/lsp/regtest: add regtest for golang/go#36951

Not sure when this got fixed, but confirmed manually and via a
regression test.

Fixes golang/go#36951

Change-Id: I7833a5d1119c34fd3fb45ea658e954eadbee750c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229129
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools