[MRG] Add run and runtmp pytest fixtures.

[ctb]

This PR adds run and runtmp fixtures, as a safer alternative to @utils.in_thisdir and @utils.in_tempdir decorators.

Briefly, decorators and test fixtures do not happily co-exist, and I am in need of running functions with decorators in a temp directory.

Both provide a RunnerContext object as do the current decorators, so to use them you can do e.g.

def test_me(runtmp):
    runtmp.sourmash(...)

    assert runtmp.last_result.status == 0
    assert expected_output in runtmp.last_result.out

[ctb]

@bluegenes @luizirber should be ready for review.

[codecov]

Codecov Report

Merging #1487 (255014e) into latest (e2dbe24) will increase coverage by 5.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1487      +/-   ##
==========================================
+ Coverage   89.76%   94.89%   +5.13%     
==========================================
  Files         123       96      -27     
  Lines       19549    15930    -3619     
  Branches     1497     1497              
==========================================
- Hits        17548    15117    -2431     
+ Misses       1775      587    -1188     
  Partials      226      226              

Flag	Coverage Δ
python	94.89% <100.00%> (+<0.01%)	⬆️
rust	?

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

Impacted Files	Coverage Δ
tests/conftest.py	74.46% <100.00%> (+3.73%)	⬆️
tests/test_sourmash.py	99.71% <100.00%> (-0.01%)	⬇️
src/core/src/index/linear.rs
src/core/src/wasm.rs
src/core/src/lib.rs
src/core/src/ffi/nodegraph.rs
src/core/tests/minhash.rs
src/core/src/index/bigsi.rs
src/core/src/sketch/hyperloglog/mod.rs
src/core/src/ffi/signature.rs
... and 19 more

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