chore(test): revert jest and babel-jest to v26

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Revert jest and babel-jest to v26 since vue-jest is still not supporting jest v27.

vue-jest issue: vuejs/vue-jest#344
vue-jest pr: vuejs/vue-jest#343

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-commenter]

Codecov Report

Merging #9377 (d483c9f) into dev (b52612b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #9377   +/-   ##
=======================================
  Coverage   65.15%   65.15%           
=======================================
  Files          94       94           
  Lines        4107     4107           
  Branches     1125     1125           
=======================================
  Hits         2676     2676           
  Misses       1153     1153           
  Partials      278      278           

Flag	Coverage Δ
unittests	65.15% <ø> (ø)

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