[gdb/testsuite] Fix gdb.tui/completion.exp for local-remote-host-notty

[gdb/testsuite] Fix gdb.tui/completion.exp for local-remote-host-notty

When running test-case gdb.tui/completion.exp with host board
local-remote-host-notty and target board native-gdbserver, I get:
...
FAIL: gdb.tui/completion.exp: completion of layout names: \
  tab completion (timeout)
...

The test-case contains a few tests that do tab completion, which requires
readline, which is unavailable with host board local-remote-host-notty.

Fix this by adding the missing check for readline_is_used.

Tested on x86_64-linux.