If the tests are specified as Jobs instead of Pods, a deadline can be specified in the JobSpec by setting the activeDeadlineSeconds field.

apiVersion: batch/v1
kind: Job
metadata:
  name: "{{ include "helm-test-timeout.fullname" . }}-test-timeout"
  labels:
{{ include "helm-test-timeout.labels" . | nindent 4 }}
  annotations:
    "helm.sh/hook": test-success
spec:
  activeDeadlineSeconds: 30
  template:
    spec:
      containers:
      # The nginx container will just launch and wait
      # and cause the test to time out
      - name: nginx
        image: nginx
      restartPolicy: Never

However, this deadline cannot be extended with the --timeout flag to helm test.