Dev test on Node.js 6 etc. on Linux, mac OS, and Windows

[brody2consult]

using the --ignore-engines flag on Yarn, as needed on Node.js 6

in order to avoid mistakes such as get-stream@5 update, which breaks installation from GitHub on Node.js version 6

NOTE: This proposal is known to fail on Node.js version 6 on mac OS due to timeout, on Windows with an error, and sometimes on Linux due to a seg fault.

I can think of the following possible workarounds in case the build does not succeed on Linux or Windows:

• just do test-integration on Node.js 6, like I proposed in test-integration on Azure Pipelines #1;
• look for a way to do yarn test with Jest in smaller parts
• run tests in sequence on Node.js 6 using the -i flag (short for --runInBand flag)
• try on any other platforms (...)