chore: modernize the samples and sample tests

[JustinBeckwith]

So you ever have one of those PRs you start where it begins small, then slowly grows out of control? Yeah, this is one of those. Sorry about that. This does a few things:

• Swaps @google-cloud/nodejs-repo-tools for execa
• Drops usage of the async, sinon, and proxyquire modules.
• Uses the assert.match assertion from chai for better failure assertions
• Rewrites the textDetection sample to use modern es6 syntax (promises, classes, etc)
• Flattens out the sample tests so they all live in one directory
• Fixes a failing test on master where I can only assume the output from the API changed
• Skips the failing automl sample test, which before this didn't run at all

[jkwlui]

I'm curious as to why automl sample tests are in this repository.. @nirupa-kumar