* Use unittest2 test runner
Since upgrading to unittest2, the test runner prints the command line to
re-run a failed test - this however relies on actually using the
unittest2 command line runner.
Previously, test files were assigned numbers - with the unittest2
runner, tests are run using suite/category names instead, like so:
```
# run the Genesis suite
build/all_tests "Genesis::``
# run all tests with "blsMapG1" in the name
build/all_tests "blsMapG1*"
# run tests verbosely
build/all_tests -v
```
A reasonable follow-up here would be to review the suite names to make
them easier to run :)
* lint
* easier-to-compare test order
* bump unittest2 (also the repo)
* renamed nimbus folder to execution_chain
* Renamed "nimbus" references to "execution_chain"
* fixed wrongly changed http reference
* delete snap types file given that it was deleted before this PR merge
* missing 'execution_chain' replacement
---------
Co-authored-by: pmmiranda <pedro.miranda@nimbus.team>
