gopls/internal/regtest: skip known flake TestGCDetails
gopls/internal/regtest: skip known flake TestGCDetails

This test is flaking at a relatively high rate on darwin and freebsd.
Skipping it is better than having a build dashboard with many known
failures.

Perhaps we could limit this skip to certain GOOS, but for now let's skip
it entirely until we understand the problem better.

For golang/go#44099

Change-Id: I58703b2db0e5768f75758080d07f9d29b8b5d661
Reviewed-on: https://go-review.googlesource.com/c/tools/+/291232
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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