Controlling kernels & providers for testing and exclusion

[takluyver]

In integrating this into the notebook, the need to test kernel machinery has come up. Finding a wide variety of kernel providers that are already installed is not ideal for testing: you want a more predictable environment.

This also reminded me that the old machinery allows for a whitelist of kernels to expose, and people may well want this in the new machinery.

1. Including/excluding kernels and kernel providers should probably be configured at the application level and passed in to KernelFinder. We could control this by exact names (spec/python3), glob patterns (spec/*) or regexes (spec/.*). If we're using patterns to select them, we may want to disallow or recommend against using special pattern characters in kernel type IDs.
2. For testing, we may want to go further, overriding the normal entrypoints discovery and specifying one or several kernel providers which may not expose entrypoints, so that the kernels available for testing can be precisely controlled, and can do strange things if needed. I'm not sure whether this should be configured by the application, or by e.g. an environment variable which the KernelFinder machinery picks up directly.