Add missing pin to two tests and re-enable for GC stress (dotnet/coreclr#24738)

Add missing pin to two tests and re-enable for GC stress (dotnet/coreclr#24738)

Test was evidently assuming that because an object that referred to an array
was pinned, so was the array. Failure was intermittent, GC is not guaranteed
to move objects or to render their former contents inaccessible.

Fixes dotnet/coreclr#24173.
Fixes dotnet/coreclr#24464.

Commit migrated from https://github.com/dotnet/coreclr/commit/9271757d2c917c5650c459a4c3fa648c2968a591