Test: spawn_instance now supports additional config.
Sentinel test files / directories layout improved.

The test now runs in a self-contained directory.
The general abstractions to run the tests in an environment where
mutliple instances are executed at the same time was extrapolated into
instances.tcl, that will be reused to test Redis Cluster.
Lock nodes.conf to avoid multiple processes using the same file.

This was a common source of problems among users.
The solution adopted is not bullet-proof as if the user deletes the
nodes.conf file manually, and starts a new instance with the same
nodes.conf file path, two instances will use the same file. However
following this reasoning the user may drop a nuclear bomb into the
datacenter as well.
clusterLoadConfig() REDIS_ERR retval semantics refined.

We should return REDIS_ERR to signal we can't read the configuration
because there is no config file only after checking errno, othewise
we risk to rewrite an existing file that was not accessible for some
other reason.
While ANET_ERR is -1, check syscall retval for -1 itself.
Redis Cluster test framework skeleton.