barectf_config_check_fail(): check that failure is caused by cfg. error

barectf_config_check_fail(): check that failure is caused by cfg. error

This patch makes barectf_config_check_fail() in `config/common.bash`
check that the output of `barectf` contains the string

    configuration: cannot create configuration

so as to ensure that the failure is _at least_ caused by a configuration
error, even if we don't know if the real failure cause matches what the
test intends to verify.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>