[MRG] use stderr for test output printing

[bluegenes]

Sets sys.stdout=sys.stderr, per #2138 (comment). Suggested as a solution to pytest-xdist not printing test output here: pytest-dev/pytest-xdist#354 (comment)

restores test output printing

@ctb is this a desired solution? Or did you find a better workaround?

[codecov]

Codecov Report

Merging #2217 (ce0f648) into latest (651ecea) will increase coverage by 7.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #2217      +/-   ##
==========================================
+ Coverage   84.66%   92.04%   +7.37%     
==========================================
  Files         131      100      -31     
  Lines       15512    11236    -4276     
  Branches     2210     2210              
==========================================
- Hits        13134    10342    -2792     
+ Misses       2085      601    -1484     
  Partials      293      293              

Flag	Coverage Δ
python	92.04% <ø> (ø)
rust	?

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

Impacted Files	Coverage Δ
src/core/src/index/sbt/mhbt.rs
src/core/tests/storage.rs
src/core/src/ffi/storage.rs
src/core/src/index/bigsi.rs
src/core/src/encodings.rs
src/core/src/sketch/nodegraph.rs
src/core/tests/minhash.rs
src/core/src/ffi/signature.rs
src/core/src/index/revindex.rs
src/core/src/ffi/minhash.rs
... and 21 more

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[ctb]

I can't think of a specific situation where it will cause problems, and we don't do anything particularly tricky with stdout/stderr outside of the script runner (runtmp.sourmash(...)) so ... let's give it a try and see!!