Previously, assertRaise call generates separate output file from each
test function in rally-cli-output-files.
It's better to get the method_name in __init__ to avoid the situation
where some method/call generate new frames. Also I added check when
after getting method_name to make sure rally instance is generated in
each test function instead of being generated once in setup().
Change-Id: Ia7b1608a3abd91caeefbaa021276d7b1a056fa16
863154ebfc