Streamline CI workflows with --ci option

[honno]

This PR helps us for #49 by introducing an option --ci. The idea of this option is to skip redundant tests for a CI scenario, i.e. signatures and type promotion which (should) be covered by the primary tests, and meta tests.

This makes the suite:

• Faster to run
• Clearer error situation i.e. no redundant signatures/type promotion errors
• Much much easier to xfail in a workflow i.e. no redundant type promotion cases

This PR is ready, but actually using --ci should wait until:

• Test type promotion in elementwise/operator tests #64 (with 1.6 release Quansight-Labs/ndindex#129 I'll want to go over the elementwise/operator tests again anyway)
• generate_stubs.py is updated to use this (a bit annoying right now, and we'll want to overhaul the file anyway after the RST conversion)