go.tools/imports: Permit fix_test.go to work pre-Go 1.4.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/156980043

imports/fix_test.go

@@ -727,7 +727,13 @@ func TestFindImportGoPath(t *testing.T) {
 	stdlib = nil
 
 	// Test against imaginary bits/bytes package in std lib
-	bytesDir := filepath.Join(goroot, "src", "bits", "bytes")
+	bytesDir := filepath.Join(goroot, "src", "pkg", "bits", "bytes")
+	for _, tag := range build.Default.ReleaseTags {
+		// Go 1.4 rearranged the GOROOT tree to remove the "pkg" path component.
+		if tag == "go1.4" {
+			bytesDir = filepath.Join(goroot, "src", "bits", "bytes")
+		}
+	}
 	if err := os.MkdirAll(bytesDir, 0755); err != nil {
 		t.Fatal(err)
 	}