Use the absolute path for the valgrind log file

This fixes the issue when the relative path is provided, which causes the process not starting correctly and the tests - failing.

RLTest/debuggers.py

@@ -25,7 +25,7 @@ def generate_command(self, logfile=None):
         if self.suppressions:
             cmd += ['--suppressions=' + self.suppressions]
         if logfile:
-            cmd += ['--log-file=' + logfile]
+            cmd += ['--log-file=' + os.path.abspath(logfile)]
         return cmd