libcni: Fixed tests that were checking error strings.
libcni: Fixed tests that were checking error strings.

Checking error strings makes these tests flaky, especially if the
error string is changed in libcni. Have gone ahead an introduced a new
error type `NoConfigsFoundError` and the Match is against the error
type making it more deterministic.

squeed/plugins,rosenhouse/cni,appc/cni,containernetworking/plugins,appc/cni,dcbw/cni,kad/cni,kad/cni,rosenhouse/cni,mcastelino/cni,containernetworking/cni,mcastelino/cni,dcbw/cni,squeed/plugins,containernetworking/plugins,containernetworking/cni