Create presubmit and end-to-end test infrastructure

[adrcunha]

Partially addresses #16.

Notes:

• Build and unit tests are currently placeholders for the real stuff (see Write unit tests #22 and Write build tests #23).
• run_buildpack_test() is the only end-to-end test, and it doesn't actually check results (see Write end-to-end tests #24).

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[imjasonh]

Can you add me and mattmoor as well?

[adrcunha]

You and Matt are already owners because you're in the top level OWNERS file. Adding you here is unnecessary redundancy.

[imjasonh]

I'm not sure it ever makes sense to run unit tests, to be honest. The intention of this repo is to be a catalog of reusable templates, and not to include any actual code.

Maybe someday we'll host the source for certain "official" builder images here too, and those might have unit tests? But for the foreseeable future, I don't expect us to have any.

[adrcunha]

Ok, the wording is not the best here, since we won't have real code. But I can see "template validation" as unit tests, for example, and would love to see them here. It would run quickly and guard the templates against breakage. WDYT?

[imjasonh]

Is the intention that this is a real repo containing buildpack-buildable source, and we'll watch the build to complete successfully? That seems like something we might just want to move into the build repo, where we already have testing infrastructure for that.

[adrcunha]

The test I wrote is basically the instructions in the README file, so I could have something in place here that could do same basic validation of a template in the repo (and show how the e2e-tests.sh script can be used). I expect that real tests will be added later, thus I filed #22, #23 and #24.

[adrcunha]

Oh, please note that without presubmit tests we can't have test jobs and thus can't have Prow to automatically merge PRs.

[evankanderson]

/assign @imjasonh