Fix the system tests on Windows

Now, that the BIND 9 builds on Windows again, we need to fix the system tests:

$ & sh.exe runall.sh $TEST_PARALLEL_JOBS
/bin/bash: ./run.sh: No such file or directory
[...]
/bin/bash: ./run.sh: No such file or directory
I:System test result summary:
I:Found 0 test results, but 95 tests were run