dnfjson: skip dnf-json tests if dnf python module isn't available

On systems where `dnf` and the Python module aren't available, skip the
unit tests that call into the `dnf-json` script.
A test flag, `-force-dnf` is added to avoid this check and run the tests
unconditionally.  This is useful for cases where the sniff check might
fail for wrong reasons or, more importantly, for cases where we want to
be sure the tests are ran and consider a missing `dnf` module to be an
error state (e.g., in CI).

internal/dnfjson/dnfjson_test.go

@@ -1,7 +1,10 @@
 package dnfjson
 
 import (
+	"flag"
 	"fmt"
+	"os"
+	"os/exec"
 	"testing"
 
 	"github.com/osbuild/osbuild-composer/internal/mocks/rpmrepo"
@@ -9,7 +12,26 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+var forceDNF = flag.Bool("force-dnf", false, "force dnf testing, making them fail instead of skip if dnf isn't installed")
+
+func dnfInstalled() bool {
+	cmd := exec.Command("python3", "-c", "import dnf")
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		fmt.Fprintf(os.Stderr, "failed to import dnf: %s\n", err.Error())
+		return false
+	}
+	return true
+}
+
 func TestDepsolver(t *testing.T) {
+	if !*forceDNF {
+		// dnf tests aren't forced: skip them if the dnf sniff check fails
+		if !dnfInstalled() {
+			t.Skip()
+		}
+	}
+
 	s := rpmrepo.NewTestServer()
 	defer s.Close()