Bug fix: add __lt__ method for JobSpec class.

Another exception is found when running "./tools/run_tests/run_tests.py
 -l c++ -c dbg" test in python3 environment. The exception complains
the "<" method is not supported when calling sorted() function to sort
the JobSpec class.

Here is the exception.

./tools/run_tests/run_tests.py -l c++ -c dbg
PASSED: make [time=1.6sec, retries=0:0]

Omitting EPOLLEXCLUSIVE tests

2020-12-01 10:49:42,122 detected port server running version 21
2020-12-01 10:49:42,182 my port server is version 21
Traceback (most recent call last):
  File "./tools/run_tests/run_tests.py", line 1919, in <module>
    build_only=args.build_only)
  File "./tools/run_tests/run_tests.py", line 1800, in _build_and_run
    one_run = set(spec for language in languages
  File "./tools/run_tests/run_tests.py", line 1801, in <genexpr>
    for spec in language.test_specs()
  File "./tools/run_tests/run_tests.py", line 425, in test_specs
    return sorted(out)
TypeError: '<' not supported between instances of 'JobSpec' and 'JobSpec'

Signed-off-by: dapeng-mi <clark_mdp@163.com>

tools/run_tests/python_utils/jobset.py

@@ -212,6 +212,9 @@ class JobSpec(object):
     def __cmp__(self, other):
         return self.identity() == other.identity()
 
+    def __lt__(self, other):
+        return self.identity() < other.identity()
+
     def __repr__(self):
         return 'JobSpec(shortname=%s, cmdline=%s)' % (self.shortname,
                                                       self.cmdline)