go/loader: fix broken tests after context removal from testing package

Updates golang/go#11811
Updates golang/go#18199

Change-Id: I2ce4615653034563a64b9c126651d2a6ce2aef50
Reviewed-on: https://go-review.googlesource.com/34274
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
1 file changed