test: Replace t.fatalf with testify require in go/vt/schemamanager

[beingnoble03]

Description

This PR replaces some t.fatalf instances with testify's require in go/vt/schemamanager

Related Issue(s)

• Chore: replace instances of t.Errorf and t.Fatalf with assert and require, respectively #15182
• Improve Unit Test Coverage #14931

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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General

• Ensure that the Pull Request has a descriptive title.
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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.78%. Comparing base (e5eb981) to head (554d472).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15600      +/-   ##
==========================================
- Coverage   65.78%   65.78%   -0.01%     
==========================================
  Files        1561     1561              
  Lines      194838   194838              
==========================================
- Hits       128171   128169       -2     
- Misses      66667    66669       +2     

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