go/build: fix build

empty is already not a nil.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5375097

src/pkg/go/build/build_test.go

@@ -40,6 +40,7 @@ var buildPkgs = []struct {
 			GoFiles:     []string{"main.go"},
 			Package:     "main",
 			Imports:     []string{"go/build/pkgtest"},
+			TestImports: []string{},
 		},
 	},
 	{
@@ -48,6 +49,7 @@ var buildPkgs = []struct {
 			CgoFiles:    []string{"cgotest.go"},
 			CFiles:      []string{"cgotest.c"},
 			Imports:     []string{"C", "unsafe"},
+			TestImports: []string{},
 			Package:     "cgotest",
 		},
 	},