test/provisioner: avoid adding duplicate file in secretGenerator

Adds a check to avoid adding a file to secretGenerator in
kustomization.yaml if it already is listed there.

Originally, in the libvirtd tests, if libvirtd_ssh_key_file was set and
TEST_PROVISION_FILE environment variable was used, it would fail if one
did not remember to reset the kustomization.yaml between tests.

Fixes: #1192

Signed-off-by: Derek Lee <derlee@redhat.com>

test/provisioner/kustomize.go

@@ -205,7 +205,11 @@ func (kh *KustomizeOverlay) SetKustomizeSecretGeneratorFile(sgName string, file
 	gs := &m.SecretGenerator[i]
 
 	newFiles := gs.GeneratorArgs.DataSources.FileSources
-	newFiles = append(newFiles, file)
+
+	if !slices.Contains(newFiles, file) {
+		newFiles = append(newFiles, file)
+	}
+
 	gs.GeneratorArgs.DataSources.FileSources = newFiles
 
 	if err = kf.Write(m); err != nil {