write more comprehensive Index.select(...) tests

[ctb]

#1420 dramatically simplifies and exposes the logic around database compatibility, and makes it possible to test the API directly in Python rather than through the sourmash command line (which is how it's mostly tested right now).

At a high level, this issue entails:

• write specific tests against select interface for key Index subclasses: LinearIndex, MultiIndex, SBT, LCA_Database
• also test that partial restrictions (e.g. just ksize) are appropriately implemented

I suggest doing things in the following order:

• adding some tests to test_index.py that directly test each branching condition in the select_signature function
• applying those same tests to LinearIndex and MultiIndex
• then doing the same thing for SBT and LCA_Database` objects

Note that while the select_signature, LinearIndex, and MultiIndex tests may seem highly redundant at the moment (because they all rely on select_signature 😉 ), it's kind of nice to have separate tests for each of these since they are used in varying circumstances and there is no guarantee that the underlying implementation will remain as it is.

[ctb]

now that we have Index protocol tests, these clearly belong there :).