Allow setting cell name

[davidbrochart]

The cell name is now of the form /tmp/ipykernel_1789/2216019953.py, which makes it hard to compare error outputs with a traceback, because the cell name is embedded in it:

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
/tmp/ipykernel_1789/2216019953.py in <module>
----> 1 foo

NameError: name 'foo' is not defined

For instance in nbclient, the new ipykernel will require rewriting our tests because we check strict output equality.
It would be nice to allow setting the cell name by passing it in an environment variable, e.g.:

export IPYKERNEL_CELL_NAME="<IPY-INPUT>"

This should be enough for tests not involving the debugger.

[blink1073]

👍🏼

[SylvainCorlay]

cc @JohanMabille does this have an impact on debugger filenames.

[JohanMabille]

Setting IPYKERNEL_CELL_NAME will definitely breaks the debugger since the computed filenames in the frontend and the backend will not be the same anymore.

[davidbrochart]

Yes, it's a convenience for testing when the debugger is not involved.