Current output:

"""
Running test: fopen()
position after fwrite: 24
position after SEEK_END: 24

Running test: fmemopen()
position after fwrite: 24
position after SEEK_END: 24

Running test: open_memstream()
position after fwrite: 24
position after SEEK_END: 4

open_memstream() final buffer size: 4
"""

Expected ouput:
"""
Running test: fopen()
position after fwrite: 24
position after SEEK_END: 24

Running test: fmemopen()
position after fwrite: 24
position after SEEK_END: 24

Running test: open_memstream()
position after fwrite: 24
position after SEEK_END: 24

open_memstream() final buffer size: 24
"""

The seek to the start of the buffer, and write data smaller than
the stream length truncates the memory stream.