1. That seeking beyond the end of a file increases the size of a file.
2. That files so extended are magically filled with null bytes.

I find no support for either in the C std, and #2 in particular turns out
not to be true on Win32 (you apparently see whatever trash happened to be
on disk).  Left #1 intact, but changed the test to check only bytes it
explicitly wrote.  Also fiddled the "expected" vs "got" failure reports
to consistently use repr (%r) -- they weren't readable otherwise.