Runs one specific example and returns an %ESpec.Example{} struct. The sequence in the following:

• evaluates 'befores' and 'lets'. 'befores' fill the map for shared, 'lets' can access shared ;
• runs 'example block';
• evaluate 'finally's' The struct has fields [status: :success, result: result] or [status: failed, error: error] The result is the value returned by example block. error is a %ESpec.AssertionError{} struct.