Add testing

[hwchiu]

• Add more testing
• Use the ip link to create virtual NIC for add/del port
• Change the file permission to testing the add bridge fail.

[codecov-io]

Codecov Report

Merging #3 into master will increase coverage by 80%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #3    +/-   ##
======================================
+ Coverage      20%   100%   +80%     
======================================
  Files           1      1            
  Lines          15     15            
======================================
+ Hits            3     15    +12     
+ Misses         11      0    -11     
+ Partials        1      0     -1

Impacted Files	Coverage Δ
ovs/ovs.go	100% <0%> (+80%)	⬆️

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