LisaTest: Replace runTest with overridden __init__

`runTest` is detected and run as a test by nosetests, since it has 'Test' in the
name. We just added it so that we could instantiate LisaTest objects
easily (like `t = FreqInvarianceTest()`). Instead, remove it and override
__init__ with a default `methodName` that doesn't have 'test' in the name.

Fixes https://github.com/ARM-software/lisa/issues/306