gopls/internal/regtest/marker: require go/packages

Add a missing testenv condition to the marker test runner.

Fixes golang/go#58396

Change-Id: I69a7fe6dfab011ba789a066e2c7677c0315eed00
Reviewed-on: https://go-review.googlesource.com/c/tools/+/466715
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

gopls/internal/lsp/regtest/marker.go

@@ -236,6 +236,9 @@ var update = flag.Bool("update", false, "if set, update test data during marker
 //   - AddImport
 //   - SelectionRanges
 func RunMarkerTests(t *testing.T, dir string) {
+	// The marker tests must be able to run go/packages.Load.
+	testenv.NeedsGoPackages(t)
+
 	tests, err := loadMarkerTests(dir)
 	if err != nil {
 		t.Fatal(err)