Use blocking sockets with timeout for xfrm_tunnel_test

Using non-blocking sockets appears to be causing flakiness due to the
tests racing against the kernel's crypto/networking infrastructure.

This commit allows these sockets to block for up to 500ms to allow time
for the crypto and routing to complete before throwing errors.

Bug: 118753529
Test: Run 20x locally, via QEMU; passing.
Change-Id: I92a39f8890244ebe918f48bcf0971e85795b2a1e