test: add unit test for pkg/util/flagext/register

[simple-xair]

Which issue(s) this PR fixes:

Fixes https://github.com/apache/incubator-seata-go/issues/1000

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 56.38%. Comparing base (efdd88a) to head (a295958).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1005      +/-   ##
==========================================
+ Coverage   56.34%   56.38%   +0.04%     
==========================================
  Files         266      266              
  Lines       17484    17484              
==========================================
+ Hits         9851     9859       +8     
+ Misses       6799     6791       -8     
  Partials      834      834              

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