minor symfony#16721 [DoctrineBridge] remove unused abstract test classes (xabbuh)

This PR was merged into the 3.0-dev branch.

Discussion
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[DoctrineBridge] remove unused abstract test classes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#15696
| License       | MIT
| Doc PR        |

Tests depending on these classes have already been removed in symfony#15696.

Commits
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0108cd4 remove unused abstract test classes

Author: fabpot