update unit testing documentation for Mac OS Mojave

Author: lorjala

UNITTESTS/README.md

@@ -11,7 +11,7 @@ Unit tests test code in small sections on a host machine. Unlike other testing t
 Please install the following dependencies to use Mbed OS unit testing. 
 
 - GNU toolchains.
-   - GCC 6 or later. We recommend you use MinGW-W64 on Windows, but any Windows port of the above GCC versions works.
+   - GCC 6 or later. We recommend you use MinGW-W64 on Windows, but any Windows port of the above GCC versions works. Default compilers can be used on Mac OS instead of GCC to shorten build times, but code coverage results can then differ.
 - CMake 3.0 or newer.
 - Python 2.7.x, 3.5 or newer.
 - Pip 10.0 or newer.
@@ -35,7 +35,8 @@ Detailed instructions for supported operating systems are below.
 #### Installing dependencies on macOS
 
 1. Install [Homebrew](https://brew.sh/).
-1. Install GCC compilers and CMake with: `brew install gcc cmake`.
+1. Install Xcode Command Line Tools with `xcode-select --install`.
+1. Install CMake with: `brew install cmake`.
 1. Install Python and Pip:
 
    ```
@@ -44,6 +45,7 @@ Detailed instructions for supported operating systems are below.
    ```
 
 1. Install Gcovr and [Mbed CLI](https://os.mbed.com/docs/latest/tools/developing-arm-mbed-cli.html) with `pip install "gcovr>=4.1" mbed-cli`.
+1. (Optional) Install GCC with `brew install gcc`.
 
 #### Installing dependencies on Windows
 
@@ -200,3 +202,6 @@ Use Mbed CLI to generate code coverage reports. For advanced use, follow these s
 
 **Problem:** Virus protection identifies files generated by CMake as malicious and quarantines the files on Windows.
 * **Solution**: Restore the false positive files from the quarantine.
+
+**Problem:** CMake compiler check fails on Mac OS Mojave when using GCC-8.
+* **Solution**: Make sure gnm (binutils) is not installed. Uninstall binutils with `brew uninstall binutils`.