apidiff: reduce file permissions in test
apidiff: reduce file permissions in test

Change-Id: I274cb27166bc5f86c8e977a381aef871d8987056
Reviewed-on: https://go-review.googlesource.com/c/exp/+/445577
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/exp,golang/exp,golang/exp