allow specification of full path of kernel file

[stevengj]

As mentioned in JuliaLang/IJulia.jl#325 (comment), it might be nice to allow the full path of the kernel spec file to be specified, so that kernels can be tested without actually being installed.

[takluyver]

I just looked into this, but it's not as easy as I had hoped. jupyter_client doesn't make it easy to specify the full path to a kernel.json file, because it expects to look that up itself. It does allow specifying kernel_cmd, which would be enough for most kernels, but we deprecated that. And I'm loath to add more input options, because when there are multiple ways to provide the same information, traitlets code gets confusing and susceptible to subtle bugs.

I'll keep thinking about what can be done.

[takluyver]

PR #41 would allow overriding how a kernel is launched, so jupyter_kernel_test could run against a kernel which is not installed through the kernelspec system.