gopls/internal/regtest: fix TestUnimportedCompletions
gopls/internal/regtest: fix TestUnimportedCompletions

Rather than downloading the module and hoping the imports cache will
be refreshed in time, start with the module downloaded and remove it.

Change-Id: I1b8e7b150d66b018d12db070d0f813a827e14cc7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/282374
Trust: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

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