This question already has an answer here:
"A testbed is a platform for conducting rigorous, transparent, and replicable testing of scientific theories, computational tools, and new technologies" (Wikipedia).
While Wikipedia seems to prefer "testbed", Merriam-Webster and Oxford prefer "test bed". I have seen "testbed" predominantly in the scientific literature I have been reading. Should I mirror the literature or add the additional space?
I have read this great Q&A which focuses mostly on hyphenation usage. Rather I am looking for guidelines for open versus closed forms. Most of the answers address this indirectly, giving descriptive rather than prescriptive answers. Many answers also allude to the differing standards of hyphenations in different contexts.