Use config path specified in solr_wrapper config instead of assuming solr/conf in rake task

[cjcolvar]

The hydra:test_server and hydra:server rake tasks only look for the solr configuration in solr/conf instead of using the path specified in the solr_wrapper config file which it also reads. This is confusing since the rake tasks uses other configuration options from the solr_wrapper config files but disregards the dir option.

https://github.com/samvera/active_fedora/blob/599fd49af7196f07d8791187e3b21756b92842e1/lib/active_fedora/rake_support.rb#L19