x/pkgsite: don't show un-runnable examples as runnable

[adg]

The example for exec.Command won't run in the playground sandbox because it depends on 'tr'.

http://golang.org/pkg/os/exec/#Command

There are other such examples. We should probably detect (somehow) that they won't run, and not display them as runnable examples on golang.org.

[jimmyfrasche]

Did a quick run through of stdlib examples.

There were some I couldn't check because #23095 seems to effect quite a few examples.

Other than os/exec, the patterns I saw were:

A lot of tests aren't playable due to files not existing. For the go/* packages this would be fixed by #23603. For the rest that use files, they appear to fall into two categories:

1. Explicitly assuming the file doesn't exist and it just won't run since it's the playground
2. Implicitly assuming the example will be executed with the current working directory set to the package directory (a lot of os.Open("testdata/afile"))

https://golang.org/pkg/crypto/x509/#example_Certificate_Verify runs locally but fails on the playground with "certificate has expired or is not yet valid", presumably because of the fake time.

https://golang.org/pkg/log/syslog/#example_Dial doesn't run because syslog.Dial isn't defined on nacl

I'm 99.999% sure this example deadlocking is a bug but will leave it up to the experts: https://golang.org/pkg/net/#example_Listener Similarly for https://golang.org/pkg/net/http/#example_Get replying "Get http://www.google.com/robots.txt: dial tcp: Protocol not available"

Some of these examples have output and some don't. If they all did that could be used to build a static list of non-runnable examples. (But then it's not clear if they're not runnable due to programmer error or playground limitations).

Given the variety and subtlety among the examples that don't run, maybe an annotation like //godoc: noplay would be the way to go?

If the examples that cannot ever be run on the playground are tagged as such, there could probably be some sort of playground trybot that makes sure examples match their output when run on the playground.