[gdb/testsuite] Declare TUI tests unsupported on msys2-ucrt64

[gdb/testsuite] Declare TUI tests unsupported on msys2-ucrt64

I tried to run the TUI tests on msys2-ucrt64, and ran into:
...
(gdb) tui enable^M
Cannot enable the TUI when output is not a terminal^M
(gdb)
...

The TUI works fine in an msys2-ucrt64 terminal, it's just the testsuite setup
that doesn't work.

Fix this by bailing out for istarget *-*-mingw* in allow_tui_tests.

Tested on x86_64-linux and msys2-ucrt64.

Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/33362
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33362