Make `@testitem`s more isolated by isolating new methods/types

[nickrobinson251]

We recently saw a case (at RAI) where a testitem that reflected over subtypes(AbstractFoo) failed if it happened to run after (and on the same worker as) a seemingly unrelated testitem that defined a new struct Foo <: AbstractFoo end

Ideally it wouldn't be possible for testitems to interact in this way.

See also #184