Fix fcntrl#tee test to use the same expected/actual buffer size.

Previously the test compared /proc/version read into expected[256]
with actual[BUFSIZ]. This CL aligns the both of the buffer sizes to
the same BUFSIZ, so that /proc/version longer than 256 bytes won't
result in a test failure.

Bug: 66872345
Test: fcntl#tee on a device with long /proc/version (Chromebook Plus)
Change-Id: I004dd2189565b0bdde1aa22a2f25fafe74560180
(cherry picked from commit 85a08ae9fead3f24daf108108257e4793a192e5f)