tests: add test with incomplete packet header

tests: add test with incomplete packet header

During developement we hit a problem where src.ctf.fs's medium returned
less data than the `minSize` parameter, when reading a faulty trace.
The trace added in this patch reproduces the problem.  It consists of a
full packet, followed by a single byte.  When trying to read a second
packet, the ItemSeqIter instance would ask the medium for at least two
bytes, but the medium would return a buffer with a single byte (the data
available until the end of file).  The fix was to make the medium
generate an error in this case.

Change-Id: I9a79360b18091f517846ca2be94fbf10fb9af349
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>