Ignore testcases' NOTICE files

$(BUILD_NATIVE_TEST) will install a test binary into /testcases
regardless of is proprietary or not.
For now, since NOTICE.xml for a system partition depends on all modules in
/testcases, "m systemimage" may trigger "installing a proprietary test".

Modules in /testcase are not actually installed, so we can ignore them
when we make a notice file.

Bug: 133454731
Test: (internal) m systemimage, and see /vendor/lib if there are
unexpected files installed.

Change-Id: I54ee51a761049b8a5ce9e3369b30b7ade6344146