[gdb/testsuite] Fix gdb.tui/tui-layout.exp for remote host

When running test-case gdb.tui/tui-layout.exp with host board
local-remote-host-notty and target board native-gdbserver, I get:
...
FAIL: gdb.tui/tui-layout.exp: terminal=dumb: execution=false: layout=asm: \
  layout asm (timeout)
...

The problem is that the test-case expects that the default "setenv TERM dumb"
has effect, which is not the case for remote host.

Fix this by skipping the test for remote host.

Tested on x86_64-linux.

gdb/testsuite/gdb.tui/tui-layout.exp

@@ -61,6 +61,10 @@ proc test_layout_or_focus {layout_name terminal execution} {
     }
 
     if {$dumb_terminal} {
+	if { [is_remote host] } {
+	    # setenv TERM dummy has no effect on remote host.
+	    return
+	}
 	gdb_test "layout $layout_name" \
 	    "Cannot enable the TUI: terminal doesn't support cursor addressing \\\[TERM=dumb\\\]"
     } else {