Rename StructureAsString to toTestString

[hackerwins]

What this PR does / why we need it:

Rename StructureAsString to toTestString.

We originally used StructureAsString to represent a method for generating a string representation, primarily for debugging purposes. However, we've now decided to use struct to represent the structure for serialization. As a result, this commit updates the method name to align with our new naming convention, which is toTestString.

Which issue(s) this PR fixes:

Related to yorkie-team/yorkie-js-sdk#561

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (107498e) 48.89% compared to head (bd577ec) 48.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #656   +/-   ##
=======================================
  Coverage   48.89%   48.89%           
=======================================
  Files          69       69           
  Lines       10076    10076           
=======================================
  Hits         4927     4927           
  Misses       4629     4629           
  Partials      520      520           

Files	Coverage Δ
pkg/document/crdt/tree.go	52.49% <100.00%> (ø)
pkg/document/time/ticket.go	88.88% <100.00%> (ø)
pkg/splay/splay.go	80.88% <100.00%> (ø)
pkg/document/crdt/array.go	33.33% <0.00%> (ø)
pkg/document/crdt/rga_tree_list.go	78.23% <0.00%> (ø)
pkg/document/crdt/text.go	59.30% <0.00%> (ø)
pkg/document/crdt/rga_tree_split.go	69.06% <0.00%> (ø)

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