Fix C++20 build

...after 40d717c7be "CppunitTest_starmath_export:
inherit from UnoApiXmlTest",

> workdir/UnpackedTarball/cppunit/include/cppunit/tools/StringHelper.h:25:9: error: overload resolution selected deleted operator '<<'
>     ost << x;
>     ~~~ ^  ~
> workdir/UnpackedTarball/cppunit/include/cppunit/TestAssert.h:74:42: note: in instantiation of function template specialization 'CppUnit::StringHelper::toString<char16_t>' requested here
>         return CPPUNIT_NS::StringHelper::toString(x);
>                                          ^
> workdir/UnpackedTarball/cppunit/include/cppunit/TestAssert.h:168:50: note: in instantiation of member function 'CppUnit::assertion_traits<char16_t>::toString' requested here
>     Asserter::failNotEqual( assertion_traits<T>::toString(expected),
>                                                  ^
> starmath/qa/extras/mmlexport-test.cxx:76:5: note: in instantiation of function template specialization 'CppUnit::assertEquals<char16_t>' requested here
>     CPPUNIT_ASSERT_EQUAL(u'\x222B', aContent[0]);
>     ^
> workdir/UnpackedTarball/cppunit/include/cppunit/TestAssert.h:333:17: note: expanded from macro 'CPPUNIT_ASSERT_EQUAL'
>   ( CPPUNIT_NS::assertEquals( (expected),              \
>                 ^
> ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/13.0.1/../../../../include/c++/13.0.1/ostream:605:5: note: candidate function [with _Traits = std::char_traits<char>] has been explicitly deleted
>     operator<<(basic_ostream<char, _Traits>&, char16_t) = delete;
>     ^

Change-Id: I4602c6876d21dc17c6b8d5dfda08a17216087aa1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/149119
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

starmath/qa/extras/mmlexport-test.cxx

@@ -10,6 +10,7 @@
 #include <sal/config.h>
 #include <test/unoapixml_test.hxx>
 
+#include <o3tl/cppunittraitshelper.hxx>
 #include <sfx2/sfxbasemodel.hxx>
 
 #include <document.hxx>