Allow runtime tests to be ran from any directory
Allow runtime tests to be ran from any directory

When trying to run the runtime tests from any directory other than
`tests`  it fails:

```
~/bpftrace$ sudo ./tests/runtime-tests.sh
python3: can't open file 'main.py': [Errno 2] No such file or directory
```

This fixes that

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace