Add explicit support for macOS.

Review Request #13507 — Created Jan. 21, 2024 and submitted — Latest diff uploaded




macOS is now a supported system for the Review Board installer. It's
been tested on macOS Ventura and Sonoma. Installation requires the
Command Line Developer Tools and Homebrew to install, but then takes
care of installing everything else needed for a full deployment.

The README has been updated to list the supported versions, and to
separate them out from the supported Linux distros.

As part of this, there's a fix for the check for brew. It no longer
outright crashes if not found.

Unit tests have been added for macOS install steps, and a silly variable
name swap was corrected. The testing datasets for x86 and arm64 had
swapped variable names. The testing results were correct, but all names
were wrong. They've now been corrected.

Unit tests passed.

Tested installing on brand-new macOS Ventura and Sonoma VMs.