testhelper: Shorten temporary dir to fix sockets on macOS

With 1af14a27 (Merge branch 'pks-test-cleanup' into 'master', 2020-11-17), we've started creating a single global test directory which is supposed to contain all generated runtime files by tests. It turns out that this breaks macOS, as sockets created in that new directory now exceed the supported maximum length of 104 characters.

Work around the issue by just shortening the name of the temporary directory.