crypto/chacha20poly1305: rename test vectors file

The previous name did not have the '_test.go' suffix and so was
always built.

Change-Id: I2d18d0ba5c863ac7f0dd0465ebafba4e022faa14
Reviewed-on: https://go-review.googlesource.com/35875
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>