syscall: fix skipping some tests on Linux
syscall: fix skipping some tests on Linux

The kernel knob /proc/sys/kernel/unprivileged_userns_clone is
only available in Debian (and Ubuntu) kernels, so if the tests
are run on e.g. Fedora, skipUnprivilegedUserClone() skips a lot
of tests.

Modify it to treat ENOENT as "it should work".

Change-Id: I959201ede139ede989cc8ab646c9bf51e0539ada
Reviewed-on: https://go-review.googlesource.com/c/go/+/417694
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Kirill Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go