[integrationtests] vlog integration test for string constants in SPARQL results

[irina-dragoste]

Write integration tests for VLog issue karmaresearch/vlog#73, in which to make sure
string constants "foo1" and "foo"^^<http://www.w3.org/2001/XMLSchema#string> are treated as the same item "foo1" in VLog SPARQL results (as well as in other inputs).