Add NORNS_DEBUG_CONFIG_FILE_OVERRIDE env variable

Added a new environment variable NORNS_DEBUG_CONFIG_FILE_OVERRIDE that
allows test programs to override the load process for the default
configuration file. When setting this variable, the test program is
expected to call either libnorns_reload_config_file() or
libnornsctl_reload_config_file() to set the configuration file for the
test explicitly.

This commit closes #3.