[CLI] ti test does not test anything in Python

[yuanming-hu]

Describe the issue

It seems that after #764, using ti test does not test anything by default.
This will essentially make CI useless in running tests and should be fixed immediately.

For example, (note 339 passed in 5.86s)

ti test        
[Taichi] mode=development
[Taichi] preparing sandbox at /tmp/taichi-dva1_20k
[Taichi] sandbox prepared
[Taichi] <dev mode>, supported archs: [cpu, cuda, opengl], commit d4904c20, python 3.6.9

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 **     Taichi Programming Language       **
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Running python tests...

=============================================================================================== test session starts ===============================================================================================
platform linux -- Python 3.6.9, pytest-5.4.1, py-1.8.0, pluggy-0.13.0
rootdir: /home/yuanming/repos/taichi
plugins: arraydiff-0.3, forked-1.1.3, remotedata-0.3.2, xdist-1.31.0, doctestplus-0.4.0, openfiles-0.4.0
gw0 [1]
.                                                                                                                                                                                                           [100%]
================================================================================================ 1 passed in 0.45s ================================================================================================
Starting 4 testing thread(s)...
=============================================================================================== test session starts ===============================================================================================
platform linux -- Python 3.6.9, pytest-5.4.1, py-1.8.0, pluggy-0.13.0
rootdir: /home/yuanming/repos/taichi
plugins: arraydiff-0.3, forked-1.1.3, remotedata-0.3.2, xdist-1.31.0, doctestplus-0.4.0, openfiles-0.4.0
gw0 [339] / gw1 [339] / gw2 [339] / gw3 [339]
........................................................................................................................................................................................................... [ 59%]
........................................................................................................................................                                                                    [100%]
=============================================================================================== 339 passed in 5.86s ===============================================================================================
Running C++ tests...
===============================================================================
All tests passed (66 assertions in 6 test cases)

[yuanming-hu]

Fixed in #774.