> Only lightly tested. In a sense, plan9/arm go is tested as well as any other platform: under the go continuous development process, every time a change is made to the compiler or runtime library, a complete test suite is run on builder machines for every supported architecture and operating system. If you look at https://build.golang.org and scroll wayyyyyyyy over to the right, the plan9/arm column refers to a set of Raspberry Pi machines run by David du Columbier and me.
In another sense, it's probably not very well tested at all: I'm not aware of any production application being run on go in Plan 9, on any machine architecture. I haven't used go seriously myself, but I find the test suite gives the OS such a brutal workout (especially with small physical memory) that it's a good way to flush out underlying Plan 9 bugs. The tests show some intermittent hard-to-reproduce failures ("flakes") on all the Plan 9 builders. Many are timing issues because the tests make assumptions about absolute speed of builder machines; but there are some "can't happen" panics during garbage collection which smell like a cache or memory barrier problem. Please don't use plan9/arm go to run your nuclear power plant just yet ...