Fix FileTestCase to really test strings with embedded NULs.
The test was intended to verify that round trip via wxFile::Write/Read()
worked even for the strings with embedded NULs but as the string wasn't
constructed correctly it didn't actually contain any NULs but was ended by the
first of them.
Fix this by using explicit length of the string as usual when dealing with
strings with embedded NULs. Also fix the conversion back to Unicode to use the
correct length.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65835
c3d73ce0-8a6f-49c7-b76d-
6d57e0e08775