types,types/json: improve test coverage

[xiekeyi98]

What problem does this PR solve?

improve test coverage.

xiekeyi@ubuntu:~/gopath/src/github.com/pingcap/tidb/types$ GO111MODULE=on go test -cover | grep coverage
coverage: 85.0% of statements
xiekeyi@ubuntu:~/gopath/src/github.com/pingcap/tidb/types$ cd json/
xiekeyi@ubuntu:~/gopath/src/github.com/pingcap/tidb/types/json$ GO111MODULE=on go test -cover | grep coverage
coverage: 85.7% of statements

We have two PRs before:

• types: improve test coverage in types/json #9697
• types: improve the code coverage #9780

But now, we need to improve it up to 85%, thus this PR be filed.

What is changed and how it works?

Add more tests to test functions.

Check List

Tests

• Unit test
• Integration test

---

This change is 

[codecov]

Codecov Report

Merging #9977 into master will decrease coverage by 0.4697%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             master      #9977        +/-   ##
================================================
- Coverage   78.0088%   77.5391%   -0.4698%     
================================================
  Files           404        403         -1     
  Lines         82024      81898       -126     
================================================
- Hits          63986      63503       -483     
- Misses        13332      13688       +356     
- Partials       4706       4707         +1

[xiekeyi98]

/run-all-tests

[xiekeyi98]

/run-all-tests

[xiekeyi98]

/run-all-tests

[winkyao]

LGTM

[crazycs520]

LGTM