There is nothing special about wxTextFile justifying having code for
dealing with non-seekable files in it, so put this code into wxFile
itself and just call its ReadAll() method from here.
This is a better fix than 41f6f17d01
originally applied (and which is going to be reverted next) as it
doesn't break wxFile::Length() for these files and also avoids
triggering an assert if the file we're trying to read was truncated by
another process in the meanwhile -- which can happen and doesn't
indicate a programming error and so shouldn't result in an assert.
Also add a unit test checking that this really works.
See #3802, #8354, #9965.
cc86de1416