test-config: dynamically build test.conf to provide absolute paths

redis config requires absolute paths for pidfile, unixsocket, and logfile
directives, and interprets relative paths incorrectly without errors.

Dynamically build the test.conf from an erb file if it is not present. This
has the side-effect of creating a single source of truth for these values.

Resolves #439
Ensure `test.conf` is regenerated when template changes.
Merge pull request #443 from redis/issue-439/use-absolute-paths-for-redis-config-directives

test-config: dynamically build test.conf to provide absolute paths