WebSearch: more complete API regression test suite

* Enriches Python API and Web API test suite with more tests related
  to /search and /record URLs with TextMARC and XMLMARC outputs (full,
  filtered fields only, subfield values only).  For example,
  `/record/85?of=tm&ot=100,700`, `/record/85?of=tm&ot=700__a`.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>