net/netip: skip some TestAddrStringAllocs tests on noopt builders

net/netip: skip some TestAddrStringAllocs tests on noopt builders

CL 403914 introduced TestAddrStringAllocs which checks that there is
only 1 alloc in Addr.String for v4-in-v6 addresses. This requires
optimizations to be enabled, otherwise there are 2 allocs. Skip the
ipv4-in-ipv6 sub-tests on noopt builders to fix failing
TestAddrStringAllocs on the noopt builders.

Change-Id: I0285264260b264b53cf822dc7cec4829e9854531
Reviewed-on: https://go-review.googlesource.com/c/go/+/405834
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>