Golang x/playground: multifile code should support tests too

The playground docs say:

If the program contains tests or examples and no main function, the service runs the tests.

This example contains tests and no main function, but it doesn't work. Looks like that's because it also contains other files.

https://play.golang.org/p/eYjtKS7J25y

I'd expect the example to run TestFoo as a test.

Comment From: bcmills

For an even smaller repro, compare:

https://play.golang.org/p/t-tmCBPADwt compiles and runs.

https://play.golang.org/p/PkTK3_aY7L8, which adds only a go.mod file, fails:

# example.com
runtime.main_main·f: function main is undeclared in the main package

Go build failed.

Comment From: bcmills

CC @ysmolsky @andybons @dmitshur

Comment From: bcmills

This support would be really useful for go test reports like the one in #32379.

Comment From: rogpeppe

Ha, I just went to report this problem and found that I reported it over 2 years ago. :rofl: Surely it's not that hard to fix?

Comment From: nikola-jokic

Can I take this one? :relaxed:

Comment From: gopherbot

Change https://go.dev/cl/462835 mentions this issue: x/playground: support multifile code with tests