Test compatibility with field libraries

[johnbillion]

• CMB2
• ACF
• Fieldmanager
• Metabox.io
• Carbon Fields
• Toolset Types

[jenkoian]

Doesn't work currently with Fieldmanager - see alleyinteractive/wordpress-fieldmanager#738

[jenkoian]

May be worth adding https://github.com/voceconnect/multi-post-thumbnails to the list as well, but that doesn't work either (voceconnect/multi-post-thumbnails#138)

[cameronjonesweb]

Doesn't seem to work with ACF in fields on options pages. Image can be set fine, but trying to retrieve the value just returns false