tests: use TestMain() instead of Init() to compile mock-dnf-json

Lets us have teardown code so we can clean up the temporary directory
where the binary was built.

internal/kojiapi/server_test.go

@@ -30,7 +30,7 @@ import (
 
 var dnfjsonPath string
 
-func init() {
+func setupDNFJSON() {
 	// compile the mock-dnf-json binary to speed up tests
 	tmpdir, err := os.MkdirTemp("", "")
 	if err != nil {
@@ -573,3 +573,10 @@ func TestJobTypeValidation(t *testing.T) {
 		test.TestRoute(t, handler, false, "GET", fmt.Sprintf("/api/composer-koji/v1/compose/%s%s", badID, path), ``, http.StatusNotFound, string(resp))
 	}
 }
+
+func TestMain(m *testing.M) {
+	setupDNFJSON()
+	defer os.RemoveAll(dnfjsonPath)
+	code := m.Run()
+	os.Exit(code)
+}