Switch to librarized version of TestingAccessWrapper

Switch to librarized version of TestingAccessWrapper

Replaces \TestingAccessWrapper (defined in core) with
\Wikimedia\TestingAccessWrapper (defined in the composer package
wikimedia/testing-access-wrapper).

See https://gerrit.wikimedia.org/r/#/q/topic:librarize-testing-access-wrapper
for downstream patches.

The core version of the class is kept around for a while to avoid
circular dependency problems.

Bug: T163434
Change-Id: I52cc257e593da3d6c3b01a909e554a950225aec8