(PUP-2701) Add PMT build metadata.json tests

Add acceptance tests to validate the deprecation of the `Modulefile`
and the usage of the `metadata.json` file when envoking the
`puppet module build` command.
(PUP-2701) Correct typos in PMT build tests

s/ng1nx/nginx/

The checksums are contained in the `checksums.json` file, so
all references should be `checksums` plural.