…ases

The previous code validated the manifests exist within the file system,
which only works when a chart is unpacked, meaning children charts
stored as tgz's in the parent chart's `charts/` directory don't exist as
individual files in the filesystem, causing validation to file.

Instead, we validate against the paths within the chart itself rather
than the filesystem, handling both chart.tgz's and charts that are
unpacked as directories in the chart/ directory.

The testing.T in the subtest wasn't being used, so subtests were
effectively not being used. Additionally, in case subtests run in
parallel, we assign tt := tt to fix closure captures on range variables,
since the variables produced by a range are re-used between iterations.

Update helm template -x to support children chart manifests in more cases