Update Documentation: Add MacOS with Make instructions for Exercism CLI (#930)

* Added a new section for using the exercism CLI to test the exercises
when running on Mac OS as. This should allow for new lerners of C++ to
be able to get up and running using the CLI without needing an IDE like
XCode setup and configured and be able to use the exercism tools
available.

* Updated formatting, spelling and grammar for the additions for the Mac
OS testing.

* Apply suggestions from code review

Co-authored-by: Victor Goff <keeperotphones@gmail.com>

---------

Co-authored-by: Tim Austin <tim@neenjaw.com>
Co-authored-by: Victor Goff <keeperotphones@gmail.com>

Author: 3 people

docs/TESTS.md

@@ -54,6 +54,7 @@ code using the appropriate command for your environment:
 * Linux with make: `make`
 * Windows with Visual Studio: Select Build / Build Solution from the menu.
 * MacOS with Xcode: Select Build from the toolbar
+* MacOS with "Unix Makefile": `make` _If using the exercism CLI app_
 
 Examples of running CMake for different environments are shown below.
 
@@ -105,4 +106,24 @@ Please download and install Xcode via the App Store or via [this link][web-xcode
 Errors similar to `The CXX compiler identification is unknown` will likely be resolved by following the [instructions to install GCC][cpp-installation-instructions] or by adding another target in Xcode as per the above paragraph.
 
 [web-xcode-download]: https://apps.apple.com/us/app/xcode/id497799835?mt=12
-[cpp-installation-instructions]: https://exercism.org/docs/tracks/cpp/installation
+[cpp-installation-instructions]: https://exercism.org/docs/tracks/cpp/installation
+
+### MacOS with Make _when using the Exercism CLI_
+#### This example specifically assists those who want to use the Exercism CLI app for configuring, testing, and submitting their C++ exercises.
+
+The generator name for CMake is `Unix Makefiles`.
+Assuming the current exercise is `bob` and we're in the exercise folder:
+
+```sh
+$ touch bob.{h,cpp}
+$ cmake -G "Unix Makefiles" .
+$ exercism test
+```
+
+In this example, we do create empty files for the implementation before running CMake, but we do **not** create an empty build directory.
+This is because the Exercism CLI uses the `.exercism/metadata.json` file in the root directory to parse the test cases.
+
+Simply type `exercism test` in the root directory to compile the tests. This should generate compile-time errors.
+Once the errors are fixed, running `exercism test` again will build and execute the tests using the Exercism CLI.
+
+[cpp-installation-instructions]: https://exercism.org/docs/tracks/cpp/installation