Refactor github workflows for extension unit tests

[jonathanpwang]

The workflows for extension tests are all the same:

• *-extension.yml

They all just do some setup and then run cargo nextest on the circuit and test dirs.
To save files and ease maintenance, we can probably make some kind of matrix that just runs a separate workflow for each extension folder.

Note it is still important that each extension test is run on a separate runner so the tests are run in parallel.

The riscv.yml should not be included in the above extension tests, because those are the official riscv test vectors.

[0x471]

Would be great if you can review the PR in your free time @jonathanpwang