When I worked for IBM 9 years ago, there was a hardware
simulator,
code name "EVE", took net-list only. I had to clone
i960/VHDL to
get net-list to use EVE. The entire testbench was
Synthesizable.
There is a book talking about "Synthesizable Testbench":
Page 20, session 2.2.5, "Principles of Verifiable RTL
Design",
Year 2000 by Lionel Benning and Harry Foster.
Wen