Unable to follow "testing language worker" docs on mac

#### Investigative information

N/A since this is local testing

#### Repro steps

Provide the steps required to reproduce the problem:

Follow these steps on a Mac:

https://github.com/Azure/azure-functions-host/wiki/Authoring-&-Testing-Language-Extensions#testing

#### Expected behavior

The host starts up as expected.

#### Actual behavior

```
> dotnet ./Microsoft.Azure.WebJobs.Script.WebHost.dll
A fatal error was encountered. The library 'libhostpolicy.dylib' required to execute the application was not found in '/Users/tylerleonhardt/Desktop/Tech/PowerShell/AzF/local/Functions.Private.2.0.11952-alpha/SiteExtensions/Functions/64bit/'.
Failed to run as a self-contained app. If this should be a framework-dependent app, specify the appropriate framework in /Users/tylerleonhardt/Desktop/Tech/PowerShell/AzF/local/Functions.Private.2.0.11952-alpha/SiteExtensions/Functions/64bit/Microsoft.Azure.WebJobs.Script.WebHost.runtimeconfig.json.
```

#### Known workarounds

None at this time.

#### Related information

I tried adding:
```
"framework": {
"name": "Microsoft.NETCore.App",
"version": "2.1.2"
}
```
or
```
"framework": {
"name": "Microsoft.NETCore.App",
"version": "2.0.0"
}
```
to the `Microsoft.Azure.WebJobs.Script.WebHost.runtimeconfig.json` but that yielded:
```
> dotnet ./Microsoft.Azure.WebJobs.Script.WebHost.dll

Unhandled Exception: System.BadImageFormatException: An attempt was made to load a program with an incorrect format.
(Exception from HRESULT: 0x8007000B)
```

These steps do work on Windows.