1) Large allocation in this test caused crash. This was not
detected by builder because builder runs tests with -test.short.

2) The command "go" for forking doesn't exist in some platforms
including android. This change uses the test binary itself which
is guaranteed to exist.

This change also adds logging of the total samples collected in
TestCPUProfileMultithreaded test that is flaky in android-arm
builder.

Change-Id: I225c6b7877d811edef8b25e7eb00559450640c42
Reviewed-on: https://go-review.googlesource.com/8131Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>