Clarify that one cannot do `from jupyter_kernel_tests import KernelTests`

[Antony Lee]

If one starts with

from jupyter_kernel_tests import KernelTests

class MyTests(KernelTests):
    ....

(instead of importing the module and using a qualified name for the class)

then running the tests will try to run both the MyTests class and the KernelTests class, and the later will error. A comment explaining this in the example test suites would save some head-scratching.

[Thomas Kluyver]

Good point. Do you have time to do a PR?

[Antony Lee]

Nah, you get to do it :)