Adding image extract Unit tests

[zerodayz]

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[zerodayz]

This PR should contain Unit tests as requested in #713, and some more.

[zerodayz]

/hold
/wip

[zerodayz]

/cc @sallyom I have created an artificial docker image intestdata folder called foobar.tar, but I have noticed that all the logic for the extraction is part of the Run() command. The Run() command expects the Docker registry. I wonder how could I inject this artificial image.

The foobar.tar contains two layers with one directory and a file and file under rootfs.
It's proper docker image:

tar -tf testdata/foobar.tar
1034fa20f0fbb9e271424c7211b4d59718a6a0d524f004be63cd9a43ee739dfc/
1034fa20f0fbb9e271424c7211b4d59718a6a0d524f004be63cd9a43ee739dfc/VERSION
1034fa20f0fbb9e271424c7211b4d59718a6a0d524f004be63cd9a43ee739dfc/json
1034fa20f0fbb9e271424c7211b4d59718a6a0d524f004be63cd9a43ee739dfc/layer.tar
a3a095226f580b68e4bd46c3a92e4b3070789376571f8ef1bb64007ab16876f7/
a3a095226f580b68e4bd46c3a92e4b3070789376571f8ef1bb64007ab16876f7/VERSION
a3a095226f580b68e4bd46c3a92e4b3070789376571f8ef1bb64007ab16876f7/json
a3a095226f580b68e4bd46c3a92e4b3070789376571f8ef1bb64007ab16876f7/layer.tar
a80a2c9c681aab344887dc54203989e213e508d13e23109ebfc129f5a049f898.json
manifest.json
repositories

Thank you!

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