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/33-size-hints.t

@@ -5,7 +5,7 @@
 #
 use i3test;
 
-my $i3 = i3("/tmp/nestedcons");
+my $i3 = i3(get_socket_path());
 
 my $x = X11::XCB::Connection->new;