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Most notably, it's missing on Windows machines. For example,
windows-amd64-race started failing consistently:

	--- FAIL: BenchmarkExecEcho
	    bench_test.go:15: could not find echo: exec: "echo": executable file not found in %PATH%

We can also reproduce this from Linux with Wine:

	$ GOOS=windows go test -bench=. -benchtime=1x -run=- -exec wine
	--- FAIL: BenchmarkExecEcho
	    bench_test.go:15: could not find echo: exec: "echo": executable file not found in %PATH%

Instead, use the "hostname" program, which is available on Windows too.
Interestingly enough, it's also slightly faster than "echo". Any program
is fine as long as it's close enough to a no-op, though.

	name        old time/op    new time/op    delta
	ExecEcho-8     422µs ± 0%     395µs ± 0%  -6.39%  (p=0.004 n=6+5)

	name        old alloc/op   new alloc/op   delta
	ExecEcho-8    6.39kB ± 0%    6.42kB ± 0%  +0.53%  (p=0.002 n=6+6)

	name        old allocs/op  new allocs/op  delta
	ExecEcho-8      36.0 ± 0%      36.0 ± 0%    ~     (all equal)

Change-Id: I772864d69979172b5cf807552c84d0e165e73051
Reviewed-on: https://go-review.googlesource.com/c/164704


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>