Avoid calling strftime() with an invalid format string

This results in an assertion from MSVC CRT implementation which was
somehow consumed by CppUnit but with the switch to Catch resulted in a
test failure.

tests/datetime/datetimetest.cpp

@@ -822,7 +822,7 @@ void DateTimeTestCase::TestTimeFormat()
         }
     }
 
-    CPPUNIT_ASSERT(wxDateTime::Now().Format("%") == "%");
+    CPPUNIT_ASSERT(wxDateTime::Now().Format("%%") == "%");
 
     wxDateTime dt;