Fix last error display in wxLogSysError().

Fix last error display in wxLogSysError().

After recent changes of wxLogXXX() functions into macros the last error was
overwritten by wxString::Format() called between the call to wxLogSysError()
and wxLog::CallDoLogNow() which called wxSysErrorCode() and so its original
value was lost and, unless the last error was specified explicitly, it always
came out as 0.

To fix this, call wxSysErrorCode() directly when calling wxLogSysError(). This
may be unnecessary (if the error is given explicitly) but there doesn't seem
to be any other way to fix it and the overhead of calling wxSysErrorCode()
shouldn't be that big.

Also add a unit test checking that wxLogSysError() behaves as expected.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61692 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775