test: don't compute knownGlobals lazily

Conditional globals like 'gc' should only be recognized when --expose_gc
is set.  The global.gc feature check works only when done eagerly, else
it lets through a leaked variable called 'gc'.
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.