Print stderr from helm test failures

[dstandish]

Previously you would not see the error message; you'd only see that the command failed. Now you'll see it.

Here's an example of what it looks like now:

test_basic_helm_chart.py:551: in test_invalid_pull_policy
    render_chart(
../../tests/charts/helm_template_generator.py:145: in render_chart
    raise HelmFailedError(result.returncode, result.args, result.stdout, result.stderr)
E   tests.charts.helm_template_generator.HelmFailedError: Helm command failed. Args: (1, ['helm', 'template', 'test-basic', '/Users/dstandish/code/airflow/chart', '--values', '/var/folders/9c/tknx7xx10qx92983y1r5djb40000gn/T/tmpbmxoj2yj', '--kube-version', '1.29.1', '--namespace', 'default'], b'', b'Error: values don\'t meet the specifications of the schema(s) in the following chart(s):\nairflow:\n- images.flower.pullPolicy: images.flower.pullPolicy must be one of the following: "Always", "Never", "IfNotPresent"\n\n')
E   Stderr: 
E   Error: values don't meet the specifications of the schema(s) in the following chart(s):
E   airflow:
E   - images.flower.pullPolicy: images.flower.pullPolicy must be one of the following: "Always", "Never", "IfNotPresent"
FAILED [ 50%]
helm_tests/airflow_aux/test_basic_helm_chart.py:544 (TestBaseChartTest.test_invalid_pull_policy[statsd])

Before you would only have seen something like this:

helm_tests/airflow_core/test_rpc_server.py:578 (TestRPCServerDeployment.test_rpc_server_security_context_legacy)
../../tests/charts/helm_template_generator.py:141: in render_chart
    result.check_returncode()
../../../../.pyenv/versions/3.11.3/lib/python3.11/subprocess.py:502: in check_returncode
    raise CalledProcessError(self.returncode, self.args, self.stdout,
E   subprocess.CalledProcessError: Command '['helm', 'template', 'release-name', '/Users/dstandish/code/airflow/chart', '--values', '/var/folders/9c/tknx7xx10qx92983y1r5djb40000gn/T/tmpcbo2q6kz', '--kube-version', '1.29.1', '--namespace', 'default', '--show-only', 'templates/rpc-server/rpc-server-deployment.yaml']' returned non-zero exit status 1.