test: Create the test folder

test/README.md

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+# Packer Black-Box Tests
+
+This folder contains tests that test Packer using a black-box approach:
+`packer` is executed directly (with whatever is on the PATH) and certain
+results are expected.
+
+Tests are run using [Bats](https://github.com/sstephenson/bats), and therefore
+Bash is required to run any tests.