gopls/internal/astutil: TestPurgeFuncBodies requires source code for std

Fixes golang/go#59349

Change-Id: Ifcc743f196ffbc91f99b0bdb175673f924cbd8ba
Reviewed-on: https://go-review.googlesource.com/c/tools/+/481275
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/astutil/purge_test.go

@@ -14,11 +14,14 @@ import (
 
 	"golang.org/x/tools/go/packages"
 	"golang.org/x/tools/gopls/internal/astutil"
+	"golang.org/x/tools/internal/testenv"
 )
 
 // TestPurgeFuncBodies tests PurgeFuncBodies by comparing it against a
 // (less efficient) reference implementation that purges after parsing.
 func TestPurgeFuncBodies(t *testing.T) {
+	testenv.NeedsGoBuild(t) // we need the source code for std
+
 	// Load a few standard packages.
 	config := packages.Config{Mode: packages.NeedCompiledGoFiles}
 	pkgs, err := packages.Load(&config, "encoding/...")