[gdb/testsuite] Handle wrap mode unsupported in gdb.tui/wrap-line.exp

When running test-case gdb.tui/wrap-line.exp on msys2-ucrt64, we run into some
fails in the CLI part of the test-case because:
...
(gdb) maint info screen
  ...
Readline wrapping mode: unsupported (terminal is not Cursor Up capable)
...

Fix this by requiring wrapping mode "readline" for that part of the test-case.

Tested on x86_64-linux and msys2-ucrt64.

Approved-By: Tom Tromey <tom@tromey.com>