Add Unit Test to check for matching error message in Generate function

Co-authored-by: Marco Franssen <marco.franssen@philips.com>

cmd/slsa-provenance/cli/generate_test.go

@@ -0,0 +1,25 @@
+package cli_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/philips-labs/slsa-provenance-action/cmd/slsa-provenance/cli"
+)
+
+const (
+	ArtifactPathError = "no value found for required flag: -artifact_path"
+)
+
+func TestErrors(t *testing.T) {
+	cli := cli.Generate()
+	err := cli.Exec(context.Background(), make([]string, 0))
+
+	if err == nil {
+		t.Error("Expected an error but did not generate one")
+	} else {
+		if err.Error() != ArtifactPathError {
+			t.Errorf("Expected error to match: %s", ArtifactPathError)
+		}
+	}
+}