Improve QBuffer autotest.

This commit fixes several issues found in the readLineBoundaries() test
function.

First, the test performed some test actions but did not perform any
verification steps to check that the outcome of those actions was
acceptable.

Second, the test didn't need to write the buffered data to a file to
verify line-by-line reading.

Third, the get/unget action was unrelated to the readLineBoundaries()
test and has been moved to a separate test function.  That test function
now tests that get/unget works at every position in a buffer, not just
at position 0.

Change-Id: Icad52ed598e94b6e05a194b9ab301d099bfc094c
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>