os/exec: don't use the echo binary for a benchmark
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)