Default cvl schema path is now same as the install path. Env varibale
CVL_SCHEMA_PATH need not be set if schema files are installed from
sonic-mgm-common.deb package. This simplifies testing.

Modified transformer to accept custom yang search path through env
variable YANG_MODELS_PATH. Falls back to default path /usr/models/yang
if this env variable is not set. Helps quick testing without install.

1) Introduced new test app module api_tests_app.go which handles paths
starting with "/api-tests:". It returns success or error based on data
in the path. Useful for testing end-to-end functionality from translib
clients, like REST server, without depending on actual app modules.

2) api_tests_app.go will compile only when build tag "test" is set.
Translib makefile sets it during translib test binary compilation only.

3) Temporarily excluded ACL app test code from translib test binary. It
will be added back later once all errors are fixed. ACL app tests can
still be run from source. Moved TestMain function and common utilities
to a new app_utils_test.go go file.

Signed-off-by: Sachin Holla <sachin.holla@broadcom.com>