Use absolute path for temporary directory in tests (#52228)

We explicitly set the path for the temporary directory to use in test
tasks, but today this path is a relative path, relative to the current
working directory of the test task. The fact that we are using a
relative path here appears to be legacy, simply leftover from the days
of the Maven build. An absolute path is preferred here, since it's
explicit and we do not have to rely on everyone resolving the path
properly relative to the working directory.