scripts/driver.py will use Valgrind to run each unittest when option
--valgrind is provided. This commit add two members to class Tracer, a
flag for option --valgrind and a string to represent the main command
for subprocess.call().

Notices:

- Remember to patch alarm function used in harness.c since Valgrind will
  inject extra code during the execution which may cause invocation of
  alarm.
  There are several ways of patching.
    - Replace alarm by isnan in executable.
    - Increase the value of variable `time_limit` declared in harness.c.
    - Remove the alarm function in harness.c.