Fix number of tokens in StringTracker merge

[lalmei]

Description

Fix a bug in string tracker with additional unit test to cover it.

General Checklist

• Tests added for this feature/bug
  if it was a bug, test must cover it.
• Conform by the style guides, by using formatter
• Documentation updated
• (optional) Please add a label to your PR

[cswarth]

LGTM!

[coveralls]

Pull Request Test Coverage Report for Build 1016099076

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.419%

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Totals
Change from base Build 1012518999:	0.0%
Covered Lines:	3489
Relevant Lines:	4081

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1016099076

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.362%

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Totals
Change from base Build 1012518999:	0.0%
Covered Lines:	3489
Relevant Lines:	4081

---

💛 - Coveralls