Write correct number of bytes in wxFile::Write(wxString).

Write correct number of bytes in wxFile::Write(wxString).

This function was broken for conversions using more than one byte per
character (e.g. UTF-16 or UTF-32) and also even for UTF-8 for strings
containing NUL bytes as it used strlen() to determine the number of bytes to
write out instead of using the really needed number.

Fix this by using the wxCharBuffer::length() method which always returns the
correct value.

Also add a wxFile unit test verifying that it can correctly read back a string
written using any of UTF-8, UTF-16 or UTF-32.

Closes #11192.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61898 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775