lib/oeqa: allow multiple layers to provide their own TEST_TARGET class

Use a python module "folder" rather than a single module within
layers to ensure multiple layers can define a TEST_TARGET class.
Current implementation using controllers.py module will only allow
a single layer to define test targets.

Add a controllers folder as well as a TestTargetLoader class whose
job is to load the given TEST_TARGET class from any number of
python modules within the oeqa/controllers/ directory of any
layer.
The only condition will be that layers will need to ensure
the TEST_TARGET class name they provide is unique otherwise there
is no guarantee which class is instantiated. a bb.warn is used
to alude to this if it happens.

(From OE-Core rev: 3f25705f4a986e06cbd397aaea52b841c1a1e054)

Signed-off-by: Sipke Vriend <sipke.vriend@xilinx.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/controllers/__init__.py

@@ -0,0 +1,3 @@
+# Enable other layers to have modules in the same named directory
+from pkgutil import extend_path
+__path__ = extend_path(__path__, __name__)