[gdb/testsuite] Fix gdb.python/tui-window-factory.exp for remote host

When running gdb.python/tui-window.exp with host board
local-remote-host-notty and target board native-gdbserver, I get:
...
FAIL: gdb.python/tui-window-factory.exp: msg_2: \
  check test_window box (box check: ul corner is l, not +)
...

The problem is that the result of Term::prepare_for_tui is not checked.

Fix this by adding the missing check.

Tested on x86_64-linux.

gdb/testsuite/gdb.python/tui-window-factory.exp

@@ -32,7 +32,10 @@ set pyfile [gdb_remote_download host \
 		${srcdir}/${subdir}/${gdb_test_file_name}.py]
 
 Term::clean_restart 24 80
-Term::prepare_for_tui
+if { ![Term::prepare_for_tui] } {
+    unsupported "TUI not supported"
+    return
+}
 
 gdb_test "source ${pyfile}" "Python script imported" \
     "import python scripts"