Enhance error message returned by cli-test-utils when sh not on PATH

[t1m0thyj]

Is your feature or enhancement request related to a problem or limitation? Please describe

In order to run CLI system tests with cli-test-utils on Windows, it is required to select the option below when installing Git Bash:

If this step is missed and Git Bash is installed with default options, then CLI system tests may fail with a cryptic error like TypeError: Cannot read properties of null (reading 'toString').

Describe your enhancement idea

To improve the experience for CLI plug-in developers, the cli-test-utils runScript method could detect if sh is missing from the PATH, and if show a more helpful error message such as:

Detected that "sh" is missing from your PATH.
If you are using Windows, ensure that Git Bash has been installed with the option to "Use Git and Unix Tools from Windows Command Prompt".

Describe alternatives you've considered

Open a Git Bash terminal to execute the npm command that runs system tests so that sh is present in the environment.

Provide any additional context

For CLI developers, we have documentation related to this in our readme:

In addition to Node.js, you must have a means to execute .sh (bash) scripts, which are required for running integration tests. On Windows, you can install "Git Bash" (bundled with the standard Git installation - check "Use Git and Unix Tools from Windows Command Prompt" installation option).

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