vim-patch:partial:9.1.1004: tests: a few termdebug tests are flaky

Problem:  tests: a few termdebug tests are flaky; test_termdebug_basic()
          and test_termdebug_config_types() may fail if there is too
          much load
Solution: Set g:test_is_flaky

Only include:
- mark Test_termdebug_basic() as flaky in oldtest termdebug plugin tests

The Test_termdebug_config_types() part does not apply here.

ebb08d5913

Co-authored-by: Christian Brabandt <cb@256bit.org>
AI-assisted: Codex

test/old/testdir/test_plugin_termdebug.vim

@@ -57,6 +57,7 @@ endfunction
 packadd termdebug
 
 func Test_termdebug_basic()
+  let g:test_is_flaky = 1
   let bin_name = 'XTD_basic'
   let src_name = bin_name .. '.c'
   call s:generate_files(bin_name)