x/playground: multifile code should support tests too

[rogpeppe]

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.

[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.

[bcmills]

CC @ysmolsky @andybons @dmitshur

[bcmills]

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

[rogpeppe]

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

[nikola-jokic]

Can I take this one? ☺️

[gopherbot]

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