[unit-test] Increase code coverage

[cfzjywxk]

Ths current unistore source code unit-test coverage is low, try to increase unit-test coverage, and add cases using fail-point utils to increase robustness, and to find problems earlier

?   	github.com/ngaut/unistore/config	[no test files]
ok  	github.com/ngaut/unistore/lockstore	10.757s	coverage: 72.4% of statements
?   	github.com/ngaut/unistore/metrics	[no test files]
?   	github.com/ngaut/unistore/pd	[no test files]
ok  	github.com/ngaut/unistore/rocksdb	2.756s	coverage: 81.6% of statements
ok  	github.com/ngaut/unistore/rowcodec	0.035s	coverage: 42.8% of statements
ok  	github.com/ngaut/unistore/tikv	0.356s	coverage: 17.9% of statements
?   	github.com/ngaut/unistore/tikv/dbreader	[no test files]
?   	github.com/ngaut/unistore/tikv/mvcc	[no test files]
ok  	github.com/ngaut/unistore/tikv/raftstore	5.653s	coverage: 13.3% of statements
?   	github.com/ngaut/unistore/unistore-server	[no test files]
?   	github.com/ngaut/unistore/util	[no test files]
ok  	github.com/ngaut/unistore/util/lockwaiter	0.106s	coverage: 98.6% of statements