macOS limits Unix socket paths to 104 bytes. The Go test TempDir
path (e.g. /var/folders/.../TestDirectConnection...679197086/001/)
easily exceeds that, causing "bind: invalid argument". Create a
short /tmp/vmtest* directory for all socket files (vnet, QMP,
dgram) so the paths stay well under the limit on every platform.
Updates #13038
Change-Id: I721d24561d1766aaa964692bc77f40a131aa9455
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>