Address PHPCS test failure

Problem/Motivation

Currently the PHPCS tests are failing.

$ vendor/bin/phpcs -s $_WEB_ROOT/modules/custom --report-junit=junit.xml --report-full --report-summary --report-source $_PHPCS_EXTRA
FILE: ...eb/modules/custom/config_merge-3487918/tests/src/Unit/ConfigMergerTest.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 6 | ERROR | [x] Use statements should be sorted alphabetically. The first
   |       |     wrong one is Drupal\Tests\UnitTestCase.
   |       |     (SlevomatCodingStandard.Namespaces.AlphabeticallySortedUses.IncorrectlyOrderedUses)
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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FILE: ..._merge-3487918/config_merge_filter/src/Plugin/ConfigFilter/MergeFilter.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 7 | ERROR | [x] Use statements should be sorted alphabetically. The first
   |       |     wrong one is Drupal\Core\Config\StorageInterface.
   |       |     (SlevomatCodingStandard.Namespaces.AlphabeticallySortedUses.IncorrectlyOrderedUses)
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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Time: 167ms; Memory: 6MB
PHP CODE SNIFFER REPORT SUMMARY
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FILE                                                            ERRORS  WARNINGS
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...config_merge_filter/src/Plugin/ConfigFilter/MergeFilter.php  1       0
...om/config_merge-3487918/tests/src/Unit/ConfigMergerTest.php  1       0
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A TOTAL OF 2 ERRORS AND 0 WARNINGS WERE FOUND IN 5 FILES
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PHPCBF CAN FIX 2 OF THESE SNIFF VIOLATIONS AUTOMATICALLY
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PHP CODE SNIFFER VIOLATION SOURCE SUMMARY
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    SOURCE                                                                 COUNT
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[x] SlevomatCodingStandard.Namespaces.AlphabeticallySortedUses.Incorrectl  2
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A TOTAL OF 2 SNIFF VIOLATIONS WERE FOUND IN 1 SOURCE
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PHPCBF CAN FIX THE 1 MARKED SOURCES AUTOMATICALLY (2 VIOLATIONS IN TOTAL)
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Steps to reproduce

Visit: https://git.drupalcode.org/issue/config_merge-3487918/-/jobs/3378719

Proposed resolution

Sort the use statements how SlevomatCodingStandard wants them.

Remaining tasks

User interface changes

API changes

Data model changes