testing/infra: Add docstrings to base test classes

It wasn't immediately obvious to me what the two Buildroot base test
classes were for, so add docstrings to explain the differences between
BRConfigTest and BRTest.

Signed-off-by: Thomas Preston <thomas.preston@codethink.co.uk>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

support/testing/infra/basetest.py

@@ -29,6 +29,7 @@ MINIMAL_CONFIG = \
 
 
 class BRConfigTest(unittest.TestCase):
+    """Test up to the configure stage."""
     config = None
     br2_external = list()
     downloaddir = None
@@ -66,6 +67,7 @@ class BRConfigTest(unittest.TestCase):
 
 
 class BRTest(BRConfigTest):
+    """Test up to the build stage and instantiate an emulator."""
     def __init__(self, names):
         super(BRTest, self).__init__(names)
         self.emulator = None