Fix incorrect glob pattern in webpack config

[amatissart]

After upgrading to react-scripts v5, we noticed unexpected Typescript errors related to test files when running the dev server.

It seems that one of the "exclude" rules has not been adapted correctly during the upgrade in #11201

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[heuperman]

I ran into this bug recently and as this PR seems to have been overlooked I've opened issue
#12453 for it in the hope of getting it merged.