pkgbase: Move all of ATF into the atf package

Parts of ATF (including the licence and some of the documentation) were
for some reason part of the tests package instead of the atf package.
Moving them to where they logically belong poses no problem since tests
already depends on atf.

PR:		294129
MFC after:	1 week
Reviewed by:	ivy
Differential Revision:	https://reviews.freebsd.org/D56158

etc/mtree/BSD.usr.dist

@@ -199,7 +199,7 @@
     sbin
     ..
     share
-        atf             tags=package=tests
+        atf             tags=package=atf
         ..
         bhyve
             gdb
@@ -236,7 +236,7 @@
         doc
             IPv6
             ..
-            atf             tags=package=tests
+            atf             tags=package=atf
             ..
             kyua            tags=package=tests
             ..