Fix getGarbageLength to return correct size

[skhugh]

What this PR does / why we need it?

Fix getGarbageLength to return correct size

Any background context you want to provide?

What are the relevant tickets?

yorkie-team/yorkie-js-sdk#714

Fixes #

Checklist

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

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (632aa5d) 78.33% compared to head (d32dddf) 78.47%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
...orkie/src/main/kotlin/dev/yorkie/util/IndexTree.kt	42.30%	11 Missing and 4 partials ⚠️
...c/main/kotlin/dev/yorkie/document/crdt/CrdtRoot.kt	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   78.33%   78.47%   +0.13%     
==========================================
  Files          63       63              
  Lines        3721     3744      +23     
  Branches      582      582              
==========================================
+ Hits         2915     2938      +23     
  Misses        500      500              
  Partials      306      306              

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[skhugh]

FYI, I've changed instrumentation test to run on a single device as it was failing too often