[libc++][test] `concat_macros.h`: Fix `TEST_HAS_NO_WIDE_CHARACTERS` syntax damage (#74987)

@mordante This was introduced by #73395 a couple of days ago.

This is causing PR checks to fail, [stage3 (generic-no-wide-characters,
libcxx-runners-8-set,
OFF)](https://github.com/llvm/llvm-project/actions/runs/7154839054/job/19484723909?pr=74254#logs):

```
In file included from /home/runner/_work/llvm-project/llvm-project/libcxx/test/std/utilities/format/format.tuple/format.functions.vformat.pass.cpp:29:
/home/runner/_work/llvm-project/llvm-project/libcxx/test/support/concat_macros.h:86:1: error: expected ')'
   86 | OutIt test_transcode(InIt first, InIt last, OutIt out_it) {
      | ^
/home/runner/_work/llvm-project/llvm-project/libcxx/test/support/concat_macros.h:80:11: note: to match this '('
   80 |   requires(std::output_iterator<OutIt, const char&> &&
      |           ^
/home/runner/_work/llvm-project/llvm-project/libcxx/test/std/utilities/format/format.tuple/format.functions.vformat.pass.cpp:63:2: error: expected unqualified-id
   63 | }
      |  ^
```