[TASK] Reset phpunit to 3.7 version

Some 6.1 tests do not work with phpunit 4.0 and above.
Instead of fixing them in 6.1 branch, we use a ext:phpunit
version that bundles phpunit 3.7.

Releases: 6.1
Resolves: #59978
Change-Id: If2d9173ef964a2f71428c495c2ac5c5171025f9c
Reviewed-on: https://review.typo3.org/31215
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel