selftests: forwarding: lib: Add require_command()

The logic for testing whether a certain command is available is used
several times in the current code base. The tests in follow-up patches
add more requirements like that.

Therefore extract the logic into a named function, require_command(),
that can be used directly from lib.sh as well as from any test that
wishes to declare dependence on some command.
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/forwarding/lib.sh

@@ -62,15 +62,18 @@ if [[ "$CHECK_TC" = "yes" ]]; then
 	check_tc_version
 fi
 
-if [[ ! -x "$(command -v jq)" ]]; then
-	echo "SKIP: jq not installed"
-	exit 1
-fi
+require_command()
+{
+	local cmd=$1; shift
 
-if [[ ! -x "$(command -v $MZ)" ]]; then
-	echo "SKIP: $MZ not installed"
-	exit 1
-fi
+	if [[ ! -x "$(command -v "$cmd")" ]]; then
+		echo "SKIP: $cmd not installed"
+		exit 1
+	fi
+}
+
+require_command jq
+require_command $MZ
 
 if [[ ! -v NUM_NETIFS ]]; then
 	echo "SKIP: importer does not define \"NUM_NETIFS\""