Fix image-optimizer tests for Turbopack (#60972)
## What?
There were a bunch of Sharp related tests failing when running the test
suite with Turbopack. The reason for this was that we automatically
detect the root of the project based on the closest lockfile (i.e.
package-lock.json). In this particular test a new package-lock.json was
added and since this suite is not yet isolated it cases problems.
This PR adds `outputFileTracingRoot` to explicitly provide the root of
the project, which ensures the right files are resolved.
Closes NEXT-2180
