Fix building tests with wxUSE_UNSAFE_WXSTRING_CONV==0

Avoid relying on implicit conversions to std::string in the tests code.

tests/mbconv/mbconvtest.cpp

@@ -1247,7 +1247,7 @@ void MBConvTestCase::TestStreamDecoder(
     {
         wxChar wc = textInputStream.GetChar();
         CPPUNIT_ASSERT_EQUAL_MESSAGE(
-            std::string(wxString::Format("At position %lu", (unsigned long)i)),
+            wxString::Format("At position %lu", (unsigned long)i).ToStdString(),
             wideBuffer[i],
             wc
         );

tests/strings/stdstrings.cpp

@@ -607,8 +607,10 @@ void StdStringTestCase::StdConversion()
 #endif
     CPPUNIT_ASSERT_EQUAL( "hello", s6 );
 
+#if wxUSE_UNSAFE_WXSTRING_CONV
     std::string s7(s4);
     CPPUNIT_ASSERT( s7 == "hello" );
+#endif
 
     wxStdWideString s8(s4);
     CPPUNIT_ASSERT( s8 == "hello" );

tests/strings/strings.cpp

@@ -1028,8 +1028,10 @@ void StringTestCase::CStrDataImplicitConversion()
     // implicit conversion of wxString is not available in STL build
 #if !wxUSE_STL
     CPPUNIT_ASSERT( CheckStrConstWChar(s, s) );
+#if wxUSE_UNSAFE_WXSTRING_CONV
     CPPUNIT_ASSERT( CheckStrConstChar(s, s) );
 #endif
+#endif
 }
 
 void StringTestCase::ExplicitConversion()

tests/strings/unicode.cpp

@@ -113,7 +113,7 @@ private:
     static std::string
     Message(size_t n, const wxString& msg)
     {
-        return std::string(wxString::Format("#%lu: %s", (unsigned long)n, msg));
+        return wxString::Format("#%lu: %s", (unsigned long)n, msg).ToStdString();
     }
 
     template <typename T>