[MRG] add some tests for Jaccard output ordering

[ctb]

This PR adds tests for optimized search Index classes (SBT and LCA_Database, currently) that ensure that we do not erroneously truncate searches. See #1925 for details.

tl;dr more tests good.

[codecov]

Codecov Report

Merging #1926 (70121b2) into latest (d1d8397) will increase coverage by 8.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1926      +/-   ##
==========================================
+ Coverage   82.93%   90.96%   +8.03%     
==========================================
  Files         125       94      -31     
  Lines       13755     9479    -4276     
  Branches     1877     1877              
==========================================
- Hits        11408     8623    -2785     
+ Misses       2075      584    -1491     
  Partials      272      272              

Flag	Coverage Δ
python	90.96% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/sourmash/lca/lca_db.py	91.30% <ø> (ø)
src/core/src/ffi/index/revindex.rs
src/core/src/ffi/nodegraph.rs
src/core/src/errors.rs
src/core/src/ffi/signature.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/lib.rs
src/core/src/encodings.rs
src/core/src/index/bigsi.rs
src/core/src/cmd.rs
... and 22 more

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[ctb]

ready for review & merge @sourmash-bio/devs

[mr-eyes]

Tests are looking good to me.