Functional tests for LoudAudioFeatureExtractor

Currently uses one manually-generated video with blank audio except between
15-20s where 1-2 sine tones are present

Only has a single property at present: SAMPLE_DIR for the path to where
sample videos are stored

TestVideoActivityFEFunctional now inherits from this instead of unittest.TestCase

Tests VAFE using a manually-created 30 seconds video with activity between
15 and 20 seconds.

Relatively quick!

Since it no longer simply produces videos this name is preferred

Also update tests to use updated module name

Uses unittest.mock.path to patch out call to matplotlib.pyplot.savefig()

TODO: better, more-granular tests of the bits of functionality here

mocks out calls to ffmpeg for obvious reasons

Includes:
 - MockInterval
 - MockFeature
 - MockSource

Gets utils coverage up to 100% (for now)

@see: 0a08af4

Covers:
 - to_json
 - move_end
 - update_duration
 - unhappy path exceptions (mostly init)

Gets test coverage for this module to 100%! (for now)

TestLoudAudioFeatureExtractor -- init only

TestSourceMedia is a mockinobject so

TODO: rename

Initialset of tests for LaughterFeatureExtractor and
RandomFeatureExtractor

This covers:
 - Source
 - SourceMedia
 - Interval