(Tracking): whitelist a "test/" directory in the Arduino library specification

[ianfixes]

To allow at least some official adoption of unit testing of Arduino, I have asked several upstream discussion areas about amending the library specification. The goal would be to standardize the test/ directory as one that does not contain any code files (as far as the compiler is concerned, when using a library as a dependency), and also so the Arduino IDE itself doesn't complain about the directory being unexpected in some way.

Here's where I raised those issues:

• https://forum.arduino.cc/index.php?topic=534624.0
• https://groups.google.com/a/arduino.cc/g/developers/c/H2SID4kN3os
• https://github.com/arduino/arduino-cli/issues/990

This ticket is just to (locally) track if/when that gets resolved.

[ianfixes]

submitted https://github.com/arduino/arduino-cli/pull/1125