Move unit tests out of texar

[gpengzhi]

resolve comments in #299

[codecov]

Codecov Report

Merging #301 into master will decrease coverage by 2.65%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
- Coverage   82.46%   79.80%   -2.66%     
==========================================
  Files         207      132      -75     
  Lines       16050    11041    -5009     
==========================================
- Hits        13235     8811    -4424     
+ Misses       2815     2230     -585     

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[gpengzhi]

CI failed due to more than one percent drop in code coverage.