runtime: don't run TestCrashDumpsAllThreads in parallel

It sometimes seems to time out on slow systems, perhaps due to
being run at the same time as a lot of other work.

Also move the code to testdata/testprog, so that we don't have to
build it separately.

I hope that this
Fixes #35356

Change-Id: I875b858fa23836513ae14d3116461e22fffd5352
Reviewed-on: https://go-review.googlesource.com/c/go/+/312510


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>