crypto: clean up subprocess-spawning tests

crypto: clean up subprocess-spawning tests

Consistently use testenv.Command and testenv.Executable, avoid redundant
testenv.Must, use testenv.CleanCmdEnv where the output is parsed, always
log the output with a preceding newline, invoke tests with -v, and
always use cmd.Environ() to preserve existing env.

Change-Id: I647ff1a8b7d162e5e8df9424030fac446a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/728641
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>