[libc++][z/OS] Disable portion of formatter.char.funsigned-char.pass.cpp for no unicode (#94044)

This PR carves out small portion of the test in subject to avoid the
following failure when unicode is not available.

```
# | Assertion failure: result == expected .../formatter.char.funsigned-char.pass.cpp 56
# |
# | Format string   ?}
# | Expected output '\x{80}'
# | Actual output   '�'
```

This was traced down to different definition of
`__code_point_view::__consume()` under macro_LIBCXX_HAS_NO_UNICODE which
is called inside `__formatter::__escape()`. The `__consume()` returns
`__ok` and code assumes that escaped sequence was already written but it
is not., thus the failure. Here is the snippen code we fall into:

```
    typename __unicode::__consume_result __result = __view.__consume();
    if (__result.__status == __unicode::__consume_result::__ok) {
      __escape = __formatter::__is_escaped_sequence_written(__str, __result.__code_point, __escape, __mark);
```