Fix flaky TestPageBufferReader2 test (#1210)

Fixes #1197

The `TestPageBufferReader2` test would fail often because of an
`off-by-1` issue. The problem can be reproduced by setting `randOffset`
to the biggest number that randInt31n may return statically like:

```
    //randOffset := int(rand.Int31n(int32(b.length)))
    randOffset := int(int32(b.length-1))
```

This makes the problem reliably reproducible as the offset is now
pointing at EOF.

Thus changing the line to this should hopefully solve the problem:
`randOffset := int(rand.Int31n(int32(b.length-1

(cherry picked from commit c51748e)

y/y_test.go

@@ -176,7 +176,7 @@ func TestPagebufferReader2(t *testing.T) {
 	require.Equal(t, n, 10, "length of buffer and length written should be equal")
 	require.NoError(t, err, "unable to write bytes to buffer")
 
-	randOffset := int(rand.Int31n(int32(b.length)))
+	randOffset := int(rand.Int31n(int32(b.length) - 1))
 	randLength := int(rand.Int31n(int32(b.length - randOffset)))
 	reader := b.NewReaderAt(randOffset)
 	// Read randLength bytes.