Co-authored-by: NShuduo Sang <sdsang@taosdata.com>

[TD-4037]<test>: add taodemo-testcase that childtable_count is less than or equal zero and num_of_records_per_req  is  equal zero

By this commit, It will close TD-3639.

It will not require test case to write *correct* datetime string to pass
a test case. One test could use seconds, milliseconds or microseconds to
check data.

That means the three tests are equivalent:

```python
.checkData(0, 0, "2020-01-01 00:00:00")
.checkData(0, 0, "2020-01-01 00:00:00.000")
.checkData(0, 0, "2020-01-01 00:00:00.000000")
```
Co-authored-by: NHuo Linhe <linhehuo@gmail.com>