test: added tests for statements methods

[yfractal]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Added tests for statements parse_column_default_constraint and sql_column_def_to_grpc_column_def

Checklist

• [x] I have written the necessary rustdoc comments.
• [x] I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

https://github.com/GreptimeTeam/greptimedb/pull/591

[codecov[bot]]

Codecov Report

Merging #622 (60fd030) into develop (c09775d) will increase coverage by 0.22%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #622      +/-   ##
===========================================
+ Coverage    86.30%   86.53%   +0.22%     
===========================================
  Files          402      402              
  Lines        50894    50978      +84     
===========================================
+ Hits         43923    44112     +189     
+ Misses        6971     6866     -105     

Flag	Coverage Δ
rust	86.53% <100.00%> (+0.22%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/datanode/src/server/grpc/ddl.rs	88.71% <100.00%> (ø)
src/sql/src/statements.rs	88.27% <100.00%> (+7.65%)	:arrow_up:
src/datanode/src/instance.rs	35.25% <0.00%> (-2.42%)	:arrow_down:
src/common/grpc/src/select.rs	95.79% <0.00%> (-1.87%)	:arrow_down:
src/frontend/src/instance/distributed.rs	75.66% <0.00%> (-1.84%)	:arrow_down:
src/datanode/src/instance/grpc.rs	71.64% <0.00%> (-1.79%)	:arrow_down:
src/catalog/src/error.rs	83.33% <0.00%> (-0.88%)	:arrow_down:
src/frontend/src/table/insert.rs	68.34% <0.00%> (-0.79%)	:arrow_down:
src/catalog/src/remote/manager.rs	69.35% <0.00%> (-0.70%)	:arrow_down:
src/frontend/src/table.rs	89.90% <0.00%> (-0.24%)	:arrow_down:
... and 31 more

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