hlsdemux: tests: add unit tests for hlsdemux

hlsdemux: tests: add unit tests for hlsdemux

Using the new GstAdaptiveDemux test framework, add tests that
exercise hlsdemux. The following tests are added:

simpleTest
A simple playlist that contains some media URLs

testMediaPlaylist
A master playlist with a variant playlist that contains media URLs

testMediaPlaylistNotFound
A master playlist that points to a missing variant playlist

testFragmentNotFound
A master playlist with a variant playlist that contains media URLs
There is a missing media file referenced from the variant playlist.

testFragmentDownloadError
A master playlist with a variant playlist that contains media URLs
During the download of one media file, the test simulates the network
connection being dropped.

testSeek
A simple test of trying to perform a seek on an HLS stream.