Simplify current tests

Simplify current tests

This patch:

* Adds the global yaml_cfg_path() fixture which returns the path to an
  hypothetical barectf YAML configuration file based on the test's
  module file and function names.

* Updates the config_fail_test() fixture to only use yaml_cfg_path()

* Updates all the test functions to only use config_fail_test() and call
  it without parameters.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>