fix error_code_test

[Code-Fight]

What this PR does:
修复error_code_test中变量为定义导致的ci失败
Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #461 (0dad230) into master (181c492) will decrease coverage by 0.05%.
The diff coverage is 35.83%.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage   40.58%   40.54%   -0.05%     
==========================================
  Files         139      141       +2     
  Lines        8751     8790      +39     
==========================================
+ Hits         3552     3564      +12     
- Misses       4910     4932      +22     
- Partials      289      294       +5     

Impacted Files	Coverage Δ
pkg/client/client.go	0.00% <0.00%> (ø)
pkg/datasource/sql/at.go	0.00% <0.00%> (ø)
pkg/datasource/sql/exec/at/at_executor.go	0.00% <0.00%> (-3.71%)	⬇️
pkg/datasource/sql/exec/at/insert_executor.go	41.37% <0.00%> (-0.28%)	⬇️
pkg/datasource/sql/exec/executor.go	0.00% <ø> (-3.45%)	⬇️
pkg/datasource/sql/tx.go	43.75% <0.00%> (ø)
pkg/datasource/sql/types/meta.go	36.84% <0.00%> (ø)
.../sql/undo/builder/mysql_delete_undo_log_builder.go	32.30% <ø> (-2.03%)	⬇️
.../sql/undo/builder/mysql_insert_undo_log_builder.go	50.24% <0.00%> (-0.25%)	⬇️
...mysql_insertonduplicate_update_undo_log_builder.go	47.90% <ø> (-0.49%)	⬇️
... and 20 more

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