add test for poisson

[enkilee]

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#378 3.6 poisson
add unit test for poisson

aistudio@jupyter-115720-6409174:~/PaddleScience$ pytest --cov=./ppsci/equation/pde test/equation/test_poisson.py
========================================================================= test session starts ==========================================================================
platform linux -- Python 3.8.8, pytest-6.2.2, py-1.10.0, pluggy-0.13.1
rootdir: /home/aistudio/PaddleScience
plugins: anyio-3.7.0, cov-4.1.0
collected 2 items                                                                                                                                                      

test/equation/test_poisson.py ..                                                                                                                                 [100%]

----------- coverage: platform linux, python 3.8.8-final-0 -----------
Name                                      Stmts   Miss  Cover
-------------------------------------------------------------
ppsci/equation/pde/__init__.py                9      0   100%
ppsci/equation/pde/base.py                   26      6    77%
ppsci/equation/pde/biharmonic.py             17     13    24%
ppsci/equation/pde/laplace.py                15     11    27%
ppsci/equation/pde/linear_elasticity.py     155    149     4%
ppsci/equation/pde/navier_stokes.py          60     53    12%
ppsci/equation/pde/normal_dot_vec.py         13      9    31%
ppsci/equation/pde/poisson.py                13      0   100%
ppsci/equation/pde/viv.py                    20     13    35%
-------------------------------------------------------------
TOTAL                                       328    254    23%


========================================================================== 2 passed in 3.86s ===========================================================================

[paddle-bot]

Thanks for your contribution!

[HydrogenSulfate]

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