all: use exec.Command.String in test logs

all: use exec.Command.String in test logs

There is no need to manually construct a human-friendly string for
a exec.Command. The String method does that for us.

Change-Id: Iff1033478000bade9cbdc079f6143a7690374258
Reviewed-on: https://go-review.googlesource.com/c/go/+/659475
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>