test cases for state change and corresponding bug fixes

test cases for state change and corresponding bug fixes

1. Created virtual iio device to test against
2. Added test case to test state change from NULL to PLAYING and backwards
3. Bug fixes-
  3.1 setting of fixated caps moved to _fixating() from _start()
  3.2 channel property list memory issue resolved
  3.3 missing sampling frequency file while setting it is now a warning
      skip to default frequency if file is not present
  3.4 More minor bug fixes
4. IIO base directories moved to header file
  These directories are modified to temporary location when testing with simulated device

V2:
- BugFix:
  - updated condition check for sampling_frequency
  - updated freeing channel list while filtering enabled channels
  - declaration following C89
- Moved fixating of caps to _fixate(). This removes unnecessary storage of fixated_caps
  and calculating fixated caps in _fixate() makes code more readable

V3:
- replaced stdio.h with glib/gprintf.h

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>