breadcrumbs:
The chrome.test extension API is defined in src/extensions/common/api/test.json, that file also has some documentation for some of the methods it defines. The same code that injects all other extension APIs in the right context (extensions::Dispatcher::UpdateBindingsForContext) also is responsible for injecting the chrome.test bindings (there is some special code there to not inject the chrome.test API unless the process has a --test-type command line flag). The src/extensions/common/api/_api_features.json file determines in what contexts the chrome.test API (and other extension APIs as well) will be injected.