Add codecov and snapshot test

[guoliu]

As title.

Notes:

• There is a warning: ts-jest[versions] (WARN) Version 23.6.0 of babel-jest installed has not been tested with ts-jest. If you're experiencing issues, consider using a supported version (>=24.0.0 <25.0.0). Please do not report issues in ts-jest if you are using unsupported versions. This warning will go away if we upgrade babel-jest to >=24.0.0. However, this will break customized plugins, see: feat(jest): bump to 24 kulshekhar/ts-jest#957 (comment) So we have to live with this error until ts-jest fix the issue.
• Jest now support typescript natively (via babel7), with many caveats. We will be able to get rid off ts-jest soon, but for the time being it's still better to use ts-jest, since typescript support in Jest/babel is still not comprehensive. See more at: What does the new major release of Jest (24) means for this library kulshekhar/ts-jest#961 and https://kulshekhar.github.io/ts-jest/user/babel7-or-ts

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@86e4508). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop      #29   +/-   ##
==========================================
  Coverage           ?   61.25%           
==========================================
  Files              ?       26           
  Lines              ?      240           
  Branches           ?       64           
==========================================
  Hits               ?      147           
  Misses             ?       76           
  Partials           ?       17

Impacted Files	Coverage Δ
components/Avatar/index.tsx	84.61% <ø> (ø)

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[guoliu]

(2) code coverage in CI

[devformatters]

Looks good. 🦊