vm: don't copy Proxy object from parent context

Make vm.runInContext() and vm.runInNewContext() stop copying the Proxy
object from the parent context into the new context when --harmony or
--harmony_proxies is in effect because it overwrites the new context's
native Proxy object.

This commit also adds a regression test for Harmony symbols.  They work
okay in the current implementation and the test should ensure it stays
that way.

These are the bug fixes from #6999.  Suggested reviewer: @tjfontaine