Previously, test7978 failed if the user did not invoke it with
GOTRACEBACK=2 already set in their environment. Environment-sensitive
test are awkward, and in this case there is a very simple workaround:
set the traceback level to the necessary value explicitly.

Change-Id: I7d576f24138aa8a41392148eae11bbeaef558573
Reviewed-on: https://go-review.googlesource.com/63275
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>