gcov / test framework: sigterm not sigkill

Otherwise, gcov data vanishes without a trace.

Add a __gcov_flush() call to the unix signal handler, under #ifdef
CLIB_GCOV. Add -DCLIB_GCOV to vpp_gcov_TAG_CFLAGS.

Change-Id: I2726e671b26dfbe7fae88f46a8207bb2b5106884
Signed-off-by: Dave Barach <dave@barachs.net>

test/framework.py

@@ -530,7 +530,7 @@ class VppTestCase(unittest.TestCase):
             cls.vpp.poll()
             if cls.vpp.returncode is None:
                 cls.logger.debug("Sending TERM to vpp")
-                cls.vpp.kill()
+                cls.vpp.terminate()
                 cls.logger.debug("Waiting for vpp to die")
                 cls.vpp.communicate()
             cls.logger.debug("Deleting class vpp attribute on %s",