misc/cgo/testsanitizer: don't run msan tests if msan doesn't work

misc/cgo/testsanitizer: don't run msan tests if msan doesn't work

Confirm that a trivial executable can build and execute using
-fsanitize=memory.

Fixes #18335 (by skipping the tests when they don't work).

Change-Id: Icb7a276ba7b57ea3ce31be36f74352cc68dc89d5
Reviewed-on: https://go-review.googlesource.com/34505
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>