x/build: missing test output in case of timeouts

[bcmills]

The test log at https://build.golang.org/log/5f3ee279eff750a1e1fbb46a5866d993c36ca7a0 ends with the following output:

##### 
Test "go_test:net" ran over 20m0s limit (20m0.036448368s)

That seems to imply that the test has deadlocked, but in order to diagnose the deadlock we need the goroutine dump, which is not present.

It is not obvious to me whether the lack of goroutine dump is a bug in the builder/buildlet sharding and logging path, in cmd/dist, or in the runtime.

Probably we should start the investigation with the builders.

CC @bradfitz @toothrot @dmitshur

[bcmills]

The examples of this sort of output seem to be numerous. I'm not sure when they started appearing.

Here's another one from over the weekend: https://build.golang.org/log/9ec791ff66e9d9d62b6f45049c0934552a176110

[gopherbot]

Change https://golang.org/cl/202477 mentions this issue: cmd/coordinator: include command output in timeout failures