Add functionality to run unittests on somechart.tgz files using external value files

[sokratix]

I suspect that when I run the unittest on a tgz file, it doesn't work as expected.

Following unittest passes with no errors although I use the wrong value (checking with regex):

helm unittest --color helmchart.tgz -v values-dev.yaml

And the same chart extracted, it fails as it supposed to do:

helm unittest --color helmchart/ -v values-dev.yaml