[gdb/testsuite] Fix gdb.tui/tui-nl-filtered-output.exp for remote host

When running test-case gdb.tui/tui-nl-filtered-output.exp with host board
local-remote-host-notty and target board native-gdbserver, I get:
...
FAIL: gdb.tui/tui-nl-filtered-output.exp: check printf output
...

The problem is that Term::enter_tui is returning 0, but the test-case doesn't
check for this, and consequently runs unsupported tests.

Fix this by adding the missing check.

Tested on x86_64-linux.