Netconf-testtool supports "schemas-dir" argument in java command line.

If the schema directory contains a file which is not readable as text then it throws "MissingSchemaSourceException: All available providers exhausted".

The exception is misleading, it should be thrown in existing validate method.

In Yang tool library, it takes only Yang type file in class FilesystemSchemaSourceCache.findFileWithNewestRev(), so condition for file type ".yang" can be added in TesttoolParameters class' validation method .