cmd/dist: let misc/cgo/testshared test timeout be scaled by slow builder

cmd/dist: let misc/cgo/testshared test timeout be scaled by slow builder

The default test timeout is 10 minutes if unspecified.

The misc/cgo/testshared test didn't use t.timeout(sec), which respects
GO_TEST_TIMEOUT_SCALE, so all builders got the default 10 minute
timeout. arm5 needs more, though, so specify 10 minutes explicitly,
which will then get scaled accordingly on slower builders.

Change-Id: I19ecfdcd9c865f2b69524484415b8fbd2852718e
Reviewed-on: https://go-review.googlesource.com/84315
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>