test: re-enabled fixedbugs/notinheap.go test

test: re-enabled fixedbugs/notinheap.go test

By using cgo.Incomplete to represent not-in-heap type.

While at it, also removing the type conversions tests, since they could
not be present without //go:notinheap pragma.

Fixes #54846

Change-Id: I5ee2a4b6498d4100c9770ed7bd62f52623e42526
Reviewed-on: https://go-review.googlesource.com/c/go/+/597536
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>