test: skip noisy "terminal mode unavailable" logs #36794

Problem:
Every CI log has a lot of noise at the end, which makes it harder to
find relevant test failures:

    Running tests from test/functional/terminal/tui_spec.lua
    ...
    T5831 TUI bg color queries the terminal for background color:
    T5832 TUI bg color triggers OptionSet from automatic background processing:
    T5833 TUI bg color sends theme update notifications when background changes #31652:
    ...
    Running tests from test/functional/ui/output_spec.lua
    ...
    WRN 2025-12-02T03:36:47.304 ui/c/T5831.28003.0 tui_handle_term_mode:223: TUI: terminal mode 2026 unavailable, state 0
    WRN 2025-12-02T03:36:47.359 ui/c/T5832.28006.0 tui_handle_term_mode:223: TUI: terminal mode 2048 unavailable, state 0
    WRN 2025-12-02T03:36:47.414 ui/c/T5833.28009.0 tui_handle_term_mode:223: TUI: terminal mode 2048 unavailable, state 0

Solution:
- Skip logging in test-mode.
    - This can be reverted later, when these logs are changed to "INFO"
      level, per this TODO comment:
      ```
      // TODO(bfredl): This is really ILOG but we want it in all builds.
      // add to show_verbose_terminfo() without being too racy ????
      WLOG("TUI: terminal mode %d unavailable, state %d", mode, state);
      ```

test/functional/testnvim.lua

@@ -610,21 +610,22 @@ function M._new_argv(...)
         assert(type(v) == 'string')
         env_opt[k] = v
       end
+      -- Set these from the environment unless the caller defined them.
       for _, k in ipairs({
-        'HOME',
         'ASAN_OPTIONS',
-        'TSAN_OPTIONS',
-        'MSAN_OPTIONS',
+        'GCOV_ERROR_FILE',
+        'HOME',
         'LD_LIBRARY_PATH',
-        'PATH',
+        'MSAN_OPTIONS',
+        'NVIM_TEST',
         'NVIM_LOG_FILE',
         'NVIM_RPLUGIN_MANIFEST',
-        'GCOV_ERROR_FILE',
-        'XDG_DATA_DIRS',
+        'PATH',
         'TMPDIR',
+        'TSAN_OPTIONS',
         'VIMRUNTIME',
+        'XDG_DATA_DIRS',
       }) do
-        -- Set these from the environment unless the caller defined them.
         if not env_opt[k] then
           env_opt[k] = os.getenv(k)
         end