gopls/internal/test: skip marker tests on darwin builders if -short

Marker tests involve a lot of Go command invocations, which themselves
involve a lot of I/O. This seems to make them remarkably slower on
legacy darwin builders. Skip if -short for now, to mitigate flakiness.

Per chat with the release team, LUCI darwin builders are significantly
faster and more stable. Since we only have to support the legacy
builders for a bit longer, a skip seems warranted.

Fixes golang/go#64473

Change-Id: I9c6d6379017943faee4c02eef32768a6f2af6551
Reviewed-on: https://go-review.googlesource.com/c/tools/+/554716
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/test/marker/marker_test.go

@@ -85,6 +85,9 @@ func TestMain(m *testing.M) {
 //   - The old tests lacked documentation, and often had failures that were hard
 //     to understand. By starting from scratch, we can revisit these aspects.
 func Test(t *testing.T) {
+	if testing.Short() && strings.HasPrefix(os.Getenv("GO_BUILDER_NAME"), "darwin-") {
+		t.Skip("golang/go#64473: skipping with -short: this test is too slow on darwin builders")
+	}
 	// The marker tests must be able to run go/packages.Load.
 	testenv.NeedsGoPackages(t)