Fix test-network-all target in out-of-tree builds

Fix test-network-all target in out-of-tree builds

The test-network-all target assumes the test-driver script lives in the
current working directory. This assumption breaks out-of-tree builds
because it actually lives in the source directory.

Automake 1.12 introduces `LOG_DRIVER` which defines the location of the
test driver script. Because Tor still supports Automake 1.11 we use the
default value of this variable directly. The default value uses the
configured shell for calling the test driver script and explicitly
prefixes the source directory.