tests: get the socket path from the X11 atom I3_SOCKET_PATH

The hard-coded socket paths are not useful when running the test suite in
parallel.

testcases/t/36-floating-ws-empty.t

@@ -9,7 +9,7 @@ BEGIN {
     use_ok('X11::XCB::Window');
 }
 
-my $i3 = i3("/tmp/nestedcons");
+my $i3 = i3(get_socket_path());
 
 my $tmp = fresh_workspace;