test/HTML/758518-entity.html exposed a bug in pushParseTest() in
runtest.c which assumed that an input file was at least 4 bytes long.
That test case is only 3 bytes, so we now take the minimum of 4 bytes
or the length of the test input. We also now use 'chunkSize' in place
of the hard-coded value '1024' later in the function.
