Add scp-without-sftp test case

[cpeel]

This builds off of @timsampsoncujo's PR in #160 (which fixes issue #157) to add the requested test.

This bug is suddenly blocking us and I'm simply trying to facilitate getting the fix in.

[pkittenis]

Thanks for the PR! Looks good, will merge as soon as the unrelated CI failures are resolved.

[codecov]

Codecov Report

Merging #188 into master will increase coverage by 1.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   87.62%   89.13%   +1.50%     
==========================================
  Files          23       23              
  Lines        1673     1675       +2     
==========================================
+ Hits         1466     1493      +27     
+ Misses        207      182      -25     

Impacted Files	Coverage Δ
pssh/clients/native/single.py	85.80% <100.00%> (+1.90%)	⬆️
pssh/clients/native/tunnel.py	88.06% <0.00%> (-1.71%)	⬇️
pssh/output.py	100.00% <0.00%> (ø)
pssh/clients/base/parallel.py	97.47% <0.00%> (+0.50%)	⬆️
pssh/clients/ssh/single.py	63.58% <0.00%> (+2.71%)	⬆️
pssh/clients/miko/single.py	92.12% <0.00%> (+4.62%)	⬆️
pssh/clients/miko/parallel.py	98.90% <0.00%> (+6.59%)	⬆️

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