net: skip TestFileFdBlocks if the "unix" network is not supported
This may fix the android failures observed starting at CL 496080, such
as the one in
https://build.golang.org/log/
7bfc4bd192e21c02a167d2d6a5649f1a2b63a8f1.
Change-Id: I4e8eaf9890da269bd1758f59a29fa2a8131d8ae6
Reviewed-on: https://go-review.googlesource.com/c/go/+/496955
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>