test: Replace uses of which with command -v

The `which` command isn't guaranteed to be available, it is just
extremely common; `command -v` is the standard way to do find an
executable program. See https://lwn.net/Articles/874049/ for more
details.

This patch replaces the uses of `which` with `command -v`, which only
appears in a couple of tests.

test/t-15-driusan_dkim/run.sh

@@ -10,7 +10,7 @@ init
 check_hostaliases
 
 for binary in dkimsign dkimverify dkimkeygen; do
-	if ! which $binary > /dev/null; then
+	if ! command -v $binary > /dev/null; then
 		skip "$binary binary not found"
 		exit 0
 	fi