Fix unit tests compilation in C++20

There is no more overload of std::ostream::operator<<() for wchar_t in
C++20, i.e. it is explicitly deleted, so we need to define some other
way of printing wchar_t out from Catch macros.

Do it by specializing Catch::StringMaker<> for it and outputting it
either as a (7 bit) ASCII character, if this is what it is, or as a
Unicode character code otherwise, as this will probably be more useful
in case of a test failure.

tests/testprec.h

@@ -55,6 +55,31 @@
     #define wxDEFAULT_MANTISSA_SIZE_3
 #endif
 
+// Many tests use wide characters or wide strings inside Catch macros, which
+// requires converting them to string if the check fails. This falls back to
+// std::ostream::operator<() by default, which never worked correctly, as there
+// never was any overload for wchar_t and so it used something else, but in C++
+// 20 this overload is explicitly deleted, so it results in compile-time error.
+//
+// Hence define this specialization to allow compiling such comparisons.
+namespace Catch
+{
+
+template <>
+struct StringMaker<wchar_t>
+{
+    static std::string convert(wchar_t wc)
+    {
+        if ( wc < 0x7f )
+            return std::string(static_cast<char>(wc), 1);
+
+        return wxString::Format("U+%06X", wc).ToStdString();
+    }
+};
+
+} // namespace Catch
+
+
 // thrown when assert fails in debug build
 class TestAssertFailure
 {