Rename convenience symlink to pytest artifacts

Rename convenience symlink to pytest artifacts

The previous symlink name convention was prone to name collisions If a
system test contained both a shell test and a pytest module of the same
name (e.g. dnstap test has both tests.sh and tests_dnstap.py), then
these would have the same convenience symlink, which could cause test
setup issues as well as confusion when examining test artifacts.

Update the naming convention to include the full pytest module name.
This results in a slightly more verbose names for shell tests (e.g.
dnstap_sh_dnstap instead of the previous dnstap_dnstap), but it removes
the chance of a collision.