st: test packed-to-unpacked transitions during iteration

The st_foreach and st_foreach_check functions support transitioning
from a packed to an unpacked state during iteration.  However, this
functionality did not get exercised by the current test suite until
now.  This should help us prevent breakage when making modifications
to st.

* ext/-test-/st/foreach/extconf.rb: new file
* ext/-test-/st/foreach/foreach.c: ditto
* test/-ext-/st/test_foreach.rb: ditto
  [Feature #10321]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47856 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* properties.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47857 b2dd03c8-39d4-4d8f-98ff-823fe69b080e