fixes for YANG unit tests

The recent expansion of distcheck scope revealed some unit test problems:

• Alpines:

../../../../../../../src/share/yang/modules/utils/check-revisions.sh: line 27: yanglint: not found
revision mismatch on keatest-module@2018-11-20.yang got 
FAIL: check-revisions.sh

• RPM distros:

yanglint: error while loading shared libraries: libyang.so.1: cannot open shared object file: No such file or directory
revision mismatch on kea-types@2019-08-12.yang got 
FAIL: check-revisions.sh

https://jenkins.aws.isc.org/job/kea-dev/job/distcheck/869/

These also happen on ut-extended, but because we run those with make check -k ..., they don't stop the job and the errors are not put in the report and not detected.