* test: distinguish nan and none

[TD-14736]

* modify test framework for datetime

* fix 2-query/math_funcs.py

* restore .gitmodules

* Revert "[TD-13408]<test>: move tests directory out"

This reverts commit 7db7bd9337cedc4cd0e3f3b16618b97d09791eaa.

* revert to make tests back

* immigrate file change in stand-alone repo to TDengine

* remove tests repository checkout
Co-authored-by: Ntangfangzhi <fztang@taosdata.com>

* [TD-13408]<test>: move tests directory out

* restore Jenkinsfile due to tests dir removal

* add --remote parameter to git submodule update

* git reset hard before checkout to branch

* call git submodule update for some submodules

* remove tests directory before checkout due to conflict

* as all branches remove tests, no need to remove it in script
Co-authored-by: Ntangfangzhi <fztang@taosdata.com>

add getVariable() to util/sql.py

add 4 testcases to query/queryWildcardLength.py

add getVariable() to util/sql.py

add 4 testcases to query/queryWildcardLength.py

leave a uncorrelated bug ---> TD-5918

add method in util/sql.py---getColNameList() and checkColNameList

add testcase query/queryCnameDisplay.py

* [TD-4752]<feature>: python connector support nanosecond.

* [TD-4752]<feature>: support nanosecond in test framework.

return integer for nanosecond in connector since python does not support nanosecond yet.

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>

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")
```

[TD-868]