internal/lsp/regtest: add a test that reproduces golang/go#38878
internal/lsp/regtest: add a test that reproduces golang/go#38878

This test partially reproduces some strange behavior with creating
new tests files. In particular, it creates a new x test in a package
that already has a test variant and adds content with a missing import.

In the test, the import is never added. However, in my own experience
debugging this in VS Code, I see the import get added but the diagnostic
never get removed. One thing at a time though...

Updates golang/go#39315

Change-Id: I724a145688b915d04abd1f21efc6f9a7506be043
Reviewed-on: https://go-review.googlesource.com/c/tools/+/235581
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

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