[settings][test] Add BASE_DIR variable

This is the root of the project and will be used to setup
the test prefix with the content of beat.examples.

beat/web/settings/test.py

@@ -53,6 +53,7 @@ LOGGING['loggers']['beat.core']['handlers'] = ['discard']
 LOGGING['loggers']['beat.web']['handlers'] = ['discard']
 LOGGING['loggers']['beat.web.utils.management.commands']['handlers'] = ['discard']
 
+BASE_DIR = os.path.dirname(os.path.abspath(__name__))
 PREFIX = os.environ.get('BEAT_TEST_PREFIX', os.path.realpath('./test_prefix'))
 ALGORITHMS_ROOT  = os.path.join(PREFIX, 'algorithms')
 PLOTTERS_ROOT    = os.path.join(PREFIX, 'plotters')