pyln: Set the prefix in TailableProc

We rely on it when logging, but didn't set it in the class, rather we
did in the sub-classes. This proved to be rather annoying to search
since it'd just fail silently and not log any output.

contrib/pyln-testing/pyln/testing/utils.py

@@ -146,6 +146,7 @@ class TailableProc(object):
         self.outputDir = outputDir
         self.logsearch_start = 0
         self.err_logs = []
+        self.prefix = ""
 
         # Should we be logging lines we read from stdout?
         self.verbose = verbose