[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.