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>
WebSearch: field-filtered MARCXML API output

* Implements field-filtered MARCXML output in Python and Web APIs.
  This was working for the TextMARC output, not for MARCXML output.
  This commit fixes the problem.  Usage: `/record/123?of=xm&ot=100,700`
  or `/search?p=ellis&of=xm&ot=100,700`.  (closes #1591)

* Adds new tests for trying to access hidden fields via the
  filtered-field API technique.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
WebSubmit: optional pdftk regression tests

* Disables WebSubmit stamping related regression tests
  in case pdftk is not available.

Reported-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  WebSubmit: optional pdftk regression tests
  WebSearch: field-filtered MARCXML API output
  WebSearch: more complete API regression test suite

* NOTE: the merge commit amends API regression tests
  as per new demo records added in the master branch.

Conflicts:
	modules/websearch/lib/websearch_regression_tests.py

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