add unit test for function (#706)

[csynineyang]

What this PR does:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.08% 🎉

Comparison is base (b9526c8) 36.96% compared to head (53b93f8) 38.04%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #739      +/-   ##
==========================================
+ Coverage   36.96%   38.04%   +1.08%     
==========================================
  Files         251      251              
  Lines       26236    26236              
==========================================
+ Hits         9697     9981     +284     
+ Misses      15497    15135     -362     
- Partials     1042     1120      +78     

Files Changed	Coverage Δ
pkg/boot/watcher.go	51.35% <100.00%> (+50.85%)	⬆️

... and 5 files with indirect coverage changes

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[csynineyang]

github.com/arana-db/arana/pkg/boot/boot.go (64.7%)
github.com/arana-db/arana/pkg/boot/discovery.go (64.6%)
github.com/arana-db/arana/pkg/boot/discovery_import.go (63.9%)
github.com/arana-db/arana/pkg/boot/discovery_watch.go (63.9%)
github.com/arana-db/arana/pkg/boot/misc.go (78.1%)
github.com/arana-db/arana/pkg/boot/watcher.go (68.1%)