Enable loading configs from file when testing
    
    In conftest.py, when the application to be used for testing
    was created, create_app() was called with the config_object
    waiverdb.config.TestConfig.
    
    The above allowed no custumization of the test configration
    by using a configuration file.
    
    At the same time, load_config was providing a way to tell, that
    the current environment is a test environment.
    
    In order to be able to use other databases than a local one when
    running the unit tests, this change sets the 'TEST' env var
    to 'true' and calls create_app() without a parameter, so that
    configuration files are considered.
    
    Setting the silent argument to True when calling from_pyfile()
    allows ignoring missing cofig files when the environment is DEV or
    TEST.
    
    Signed-off-by: Hunor Csomortáni <csomh@redhat.com>