pkgbase: fix inclusion of tests in ssh, bsnmp, clibs-dev

Currently, files that belong in the tests package are included in the
ssh, bsnmp, and clibs-dev packages:

ssh.plist
24:@dir(root,wheel,0755,) /usr/tests/secure/libexec
25:@(root,wheel,0444,) /usr/tests/secure/libexec/Kyuafile

bsnmp.plist
82:@dir(root,wheel,0755,) /usr/tests/lib/libbsnmp
83:@(root,wheel,0444,) /usr/tests/lib/libbsnmp/Kyuafile
84:@(root,wheel,0555,) /usr/tests/lib/libbsnmp/bsnmpd_test

clibs-dev.plist
2518:@dir(root,wheel,0755,) /usr/tests/lib/csu
2519:@(root,wheel,0444,) /usr/tests/lib/csu/Kyuafile

This is caused by the PACKAGE=foo assignment in foo/Makefile.inc which
overrides the default PACKAGE?=tests in bsd.test.mk.

To fix this, instead use PACKAGE?=foo in foo/Makefile.inc and set
PACKAGE=tests in foo/tests/Makefile.

PR: 249144
Reviewed by: bapt, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47025

(cherry picked from commit 3a56015a2f5d630910177fa79a522bb95511ccf7)