Add test utility to wrap pytest.main incantation

[areusch]

Over the past year or two I've been encouraging the use of pytest.main in TVM unittests to make it easier to run a single test case without requiring developers to let pytest go through collection of hundreds of unittests:

if __name__ == '__main__':
  sys.exit(pytest.main([__file__] + sys.argv[1:]))

This is quite an earful and it's hard to get right. It'd be better to reduce this to:

if __name__ == '__main__':
  tvm.testing.main()

This GH issue tracks that task.