[MRG] fix gather against signatures with abundances, in a new place.

[ctb]

In #1519, we fixed a bug #1518 introduced in #1370.

Unfortunately, I borked the test, and the fix wasn't tested with --no-prefetch. This PR fixes the tests in the old fix and then fixes the remaining bug.

@bluegenes @taylorreiter ready for review and merge!

[codecov]

Codecov Report

Merging #1528 (1f95a92) into latest (9712009) will increase coverage by 4.99%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1528      +/-   ##
==========================================
+ Coverage   90.20%   95.20%   +4.99%     
==========================================
  Files         126       99      -27     
  Lines       21192    17492    -3700     
  Branches     1594     1594              
==========================================
- Hits        19117    16653    -2464     
+ Misses       1844      608    -1236     
  Partials      231      231              

Flag	Coverage Δ
python	95.20% <100.00%> (ø)
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/index.py	97.63% <100.00%> (ø)
tests/test_sourmash.py	99.72% <100.00%> (ø)
src/core/src/index/linear.rs
src/core/src/ffi/mod.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/errors.rs
src/core/src/sketch/nodegraph.rs
src/core/src/index/storage.rs
src/core/src/index/sbt/mod.rs
... and 19 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9712009...1f95a92. Read the comment docs.

[taylorreiter]

LGTM, sorry I missed this in the last PR!