test: unbreak performance tests

In 8e7fb88237aedea22 Jani replaced the use of $(pwd -P) to find the
TEST_DIRECTORY in order to better support out of tree
builds. Unfortunately the performance-tests need a different value for
the variable and were thus broken.

This commit splits out the setting of this variable for the two sets
of tests.  Performance tests still don't work out of tree, because
the handling of the downloaded corpus needs to be updated.