syscall: skip TestUnshare if there's nothing more to unshare

Tests that need to use the internet are expected not to run when -short
test flag is set, and the Go build system automatically catches when a
test forgets that. It does this by unsharing all real network interfaces
and leaving only a loopback interface in a new network namespace.

TestUnshare tests that a process started with CLONE_NEWNET unshare flag
has fewer network interfaces than before. Of course, if /proc/net/dev
starts out with a single loopback interface, the test would fail with
a false positive:

=== RUN TestUnshare
exec_linux_test.go:139: Got 3 lines of output, want <3
--- FAIL: TestUnshare (0.00s)

Give the test what it wants: a skip when the environment doesn't meet
the minimum requirements for the test, and more useful log output if
it fails.

Change-Id: I6b9c29d88ce725e640a7ee86c7e1be9761f21b02
Reviewed-on: https://go-review.googlesource.com/c/go/+/513762
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

0 files changed, 0 insertions, 0 deletions