encoding/gob: add Encode-Decode Int slices tests

encoding/gob: add Encode-Decode Int slices tests

Tinkering with the gob package shows that is currently possible to
*completely destroy* Int slices encoding without triggering a single
test failure.

The various encInt{8,16,32,64}Slice methods are only called during the
execution of the GobMapInterfaceEncode test, which only encodes a few
slices of length exactly 1 and then just checks that the error
returned by Encode is nil (without trying to Decode back the data).

This patch adds a few tests for signed integer slices encoding.

Change-Id: Ifaaee2f32132873118b241f79aa8203e4ad31416
Reviewed-on: https://go-review.googlesource.com/38066
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>