Fix test framework keepalive

The pipe used for sending keepalive messages was being added to an
instance of KeepAliveReporter and then used by the class itself. This
worked in the past but doesn't anymore. Fix the issue by adding the pipe
to the class instead of an instance.

Change-Id: If7cdca2de23ca78448e80569b155e9e29e81ff94
Signed-off-by: juraj.linkes <juraj.linkes@pantheon.tech>

test/framework.py

@@ -1255,8 +1255,7 @@ class VppTestRunner(unittest.TextTestRunner):
         super(VppTestRunner, self).__init__(sys.stdout, descriptions,
                                             verbosity, failfast, buffer,
                                             resultclass)
-        reporter = KeepAliveReporter()
-        reporter.pipe = keep_alive_pipe
+        KeepAliveReporter.pipe = keep_alive_pipe
 
         VppTestResult.test_framework_result_pipe = result_pipe