tests: fix error in VppDiedError exception

Discovered running test-debug job in CI.

- fix missing paren () around format value.
Type: test

Change-Id: Iebddd3035a435f8ad1cb1d6fa4e8e8c2d4ddaf96
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/framework.py

@@ -114,8 +114,8 @@ class VppDiedError(Exception):
         msg = "VPP subprocess died %sunexpectedly with return code: %d%s." % (
             in_msg,
             self.rv,
-            ' [%s]' % self.signal_name if
-            self.signal_name is not None else '')
+            ' [%s]' % (self.signal_name if
+                       self.signal_name is not None else ''))
         super(VppDiedError, self).__init__(msg)