[test] Add coverage threshold to make codecov not fail so easily

[archibate]

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According to https://docs.codecov.io/docs/commit-status.

[codecov]

Codecov Report

Merging #1246 into master will decrease coverage by 0.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1246      +/-   ##
==========================================
- Coverage   83.97%   83.39%   -0.59%     
==========================================
  Files          18       18              
  Lines        3258     3258              
  Branches      608      608              
==========================================
- Hits         2736     2717      -19     
- Misses        387      402      +15     
- Partials      135      139       +4     

Impacted Files	Coverage Δ
python/taichi/lang/common_ops.py	87.29% <0.00%> (-3.32%)	⬇️
python/taichi/lang/matrix.py	82.60% <0.00%> (-2.20%)	⬇️

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[k-ye]

The change LGTM, but I guess we need to agree on the threshold? (I don't have a good sense on this, but personally I'm fine relaxing it a bit more)

[yuanming-hu]

Cool! 2% sounds like a reasonable threshold. Let's see how it works. Thank you!