testsuite: Look for .ovsschema files in source dir as well as build dir.

testsuite: Look for .ovsschema files in source dir as well as build dir.

When a distribution is built with "make dist", the .ovsschema files are
included as part of it, so that the builder does not have to have Python
installed.  However in that case the distributed .ovsschema files are in
the source dir instead of the build dir.  The testsuite always expected
them in the latter directory.  This commit makes it look for them in both
places.