archive/zip: actually test uncompressed size

Fixes #7292.

LGTM=dsymonds
R=dsymonds
CC=golang-codereviews
https://golang.org/cl/61650046

src/pkg/archive/zip/reader_test.go

@@ -355,8 +355,6 @@ func readTestFile(t *testing.T, zt ZipTest, ft ZipTestFile, f *File) {
 
 	testFileMode(t, zt.Name, f, ft.Mode)
 
-	size0 := f.UncompressedSize
-
 	var b bytes.Buffer
 	r, err := f.Open()
 	if err != nil {
@@ -364,10 +362,6 @@ func readTestFile(t *testing.T, zt ZipTest, ft ZipTestFile, f *File) {
 		return
 	}
 
-	if size1 := f.UncompressedSize; size0 != size1 {
-		t.Errorf("file %q changed f.UncompressedSize from %d to %d", f.Name, size0, size1)
-	}
-
 	_, err = io.Copy(&b, r)
 	if err != ft.ContentErr {
 		t.Errorf("%s: copying contents: %v (want %v)", zt.Name, err, ft.ContentErr)
@@ -377,6 +371,14 @@ func readTestFile(t *testing.T, zt ZipTest, ft ZipTestFile, f *File) {
 	}
 	r.Close()
 
+	size := int(f.UncompressedSize)
+	if size == 1<<32-1 {
+		size = int(f.UncompressedSize64)
+	}
+	if g := b.Len(); g != size {
+		t.Errorf("%v: read %v bytes but f.UncompressedSize == %v", f.Name, g, size)
+	}
+
 	var c []byte
 	if ft.Content != nil {
 		c = ft.Content