tstest/natlab/vmtest: use short paths for Unix sockets

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>

tstest/natlab/vmtest/qemu.go

@@ -162,7 +162,7 @@ func (e *Env) startCloudQEMU(n *Node) error {
 	}
 
 	logPath := filepath.Join(e.tempDir, n.name+".log")
-	qmpSock := filepath.Join(e.tempDir, n.name+"-qmp.sock")
+	qmpSock := filepath.Join(e.sockDir, n.name+"-qmp.sock")
 
 	args := []string{
 		"-machine", "q35",