[MRG] fix _location SNAFU in SBT; add tests

[ctb]

Fixes problem caused by overly hasty merge in #1373.

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1380 (7f601c1) into latest (b6c28d8) will increase coverage by 5.49%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1380      +/-   ##
==========================================
+ Coverage   88.64%   94.14%   +5.49%     
==========================================
  Files         123       96      -27     
  Lines       18267    14659    -3608     
  Branches     1409     1409              
==========================================
- Hits        16193    13800    -2393     
+ Misses       1835      621    -1214     
+ Partials      239      238       -1     

Flag	Coverage Δ
python	94.14% <100.00%> (+0.24%)	⬆️
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/sbt.py	84.30% <100.00%> (+0.26%)	⬆️
tests/test_sbt.py	100.00% <100.00%> (+5.75%)	⬆️
src/core/src/ffi/hyperloglog.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/index/search.rs
src/core/src/ffi/signature.rs
src/core/src/encodings.rs
src/core/tests/test.rs
src/core/src/ffi/nodegraph.rs
src/core/src/index/sbt/mod.rs
... and 21 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 b6c28d8...7f601c1. Read the comment docs.