add and update zip util tests with ZipReadCloser coverage

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

internal/file/test-fixtures/generate-zip-fixture-from-source-dir.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -eux
+
+# $1 —— absolute path to destination file, should end with .zip, ideally
+# $2 —— absolute path to directory from which to add entries to the archive
+
+pushd "$2" && find . -print | zip "$1" -@ && popd