[gdb/testsuite] Require allow_tui_tests in gdb.tui/tui-missing-src.exp

[gdb/testsuite] Require allow_tui_tests in gdb.tui/tui-missing-src.exp

When running test-case gdb.tui/tui-missing-src.exp on msys2-ucrt64, I run into
an error due to some filename mismatch.

I could try to fix this, but a bit later the test-case would bail out at
Term::enter_tui because allow_tui_tests returns 0.

Instead, fix this by adding an early require of allow_tui_tests.

Tested on x86_64-linux and msys2-ucrt64.

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