Add cocotb testbench runner

[jsouter]

Rebasing Jacob's work on the cocotb testbench runner on master and fixing up the git history a bit (WIP).
I've added a few commits on top to use the pytest runner and upload "cobertura" style coverage from nvc to codecov.

Opening this PR as a draft while I work on fixing up the CI etc

[jsouter]

Note to self: codecov step still shows as passing even when it fails to upload

==> Running command '/__w/_actions/codecov/codecov-action/v4/dist/codecov create-commit'
/__w/_actions/codecov/codecov-action/v4/dist/codecov create-commit --git-service github -C 63b56592eb3e2eb435da254df7f86976bd4e2df2
[PYI-22035:ERROR] Failed to load Python shared library '/tmp/_MEI4gkdDU/libpython3.11.so.1.0': dlopen: /lib64/libm.so.6: version `GLIBC_2.[29](https://github.com/PandABlocks/PandABlocks-FPGA/actions/runs/12825589516/job/35763849910?pr=222#step:7:31)' not found (required by /tmp/_MEI4gkdDU/libpython3.11.so.1.0)

Currently using the container from this PR PandABlocks/PandABlocks-rootfs#67, may need to change the python version or something.

The coverage file generated uses absolute paths which only work inside the container (prefixed by /repos/PandABlocks-FPGA) so using a sed command to make these relative so we can hopefully get line by line breakdowns of coverage.

edit: apparently when this was working the codecov-action was running on ubuntu-latest instead of inside the container, and the cocotb tests were done inside a docker run command...

[codecov]

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