Integration tests: add license holder

Merged from http://go/wvgerrit/146154

Many integration tests require a license from a license server. This
CL creates a helper class to fetch, load, and hold a license.

Test: ./build_and_run_all_unit_tests.sh

Bug: 194342800
Bug: 194342778
Change-Id: I0de7bcab4db1b365f074bad29fc157a5eca135d8

libwvdrmengine/cdm/test/integration-test.mk

@@ -17,6 +17,7 @@ LOCAL_SRC_FILES := \
     ../core/test/config_test_env.cpp \
     ../core/test/fake_provisioning_server.cpp \
     ../core/test/http_socket.cpp \
+    ../core/test/license_holder.cpp \
     ../core/test/license_request.cpp \
     ../core/test/test_base.cpp \
     ../core/test/test_printers.cpp \