Add missing libnss_testX.so requirement for tst-nss-test3.

Sometimes tst-nss-test3 fails with:
error: test-container.c:386: unable to open .../nss/libnss_test1.so for reading

The test tst-nss-test3 which runs in a container needs
libnss_test[12].so. (see e.g. tst-nss-test3.script).
Before this test was moved from tests to tests-container variable,
the requirement was met.  Thus this patch adds this requirement
also for tests in tests-container.

ChangeLog:

	* nss/Makefile (tst-nss-test3.out): New rule.