[Patch] Tweak unbuffered overflow, provide unbuffered seek testcases

[Benjamin Kosnik]

>There are *dozens* of testcases of this type in the testsuite!

Incorrect test cases should definitely be fixed as you find them. For
this particular case, you'll have to flush or seek inbetween the output
and input.

Please use .tst as the extension for input files that are read/write and
can be modified by test files. They eventually get compared to a
corresponding .txt file that should match the final .tst contents.
(Don't worry about this last bit at the moment, just get the extension
right for the files that you add.)

-benjamin