syscall: skip unshare tests if mount syscall is not available
CL 513779 added crude skips for tests that couldn't work when run under
'unshare --net --map-root-user' as used by the current iteration of the
no-network check in LUCI. Bryan suggested a more targeted way to detect
when the environment is insufficient, which makes it possible to remove
the builder-specific skip and its slightly incorrect explaining comment.
Updates #30612.
Change-Id: I0de79f44ab94d7f1018384c2e959ca7df3a1b0ae
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-linux-amd64-longtest-race
Reviewed-on: https://go-review.googlesource.com/c/go/+/526835
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com>