Clean-up R tmp directories in tests

[lbartnik]

• a number of test cases need additional clean-up calls to make sure all Rtmp* directories are removed after use

[codecov-io]

Codecov Report

Merging #45 into master will decrease coverage by 1.52%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage      80%   78.47%   -1.53%     
==========================================
  Files          11       11              
  Lines         655      655              
==========================================
- Hits          524      514      -10     
- Misses        131      141      +10

Impacted Files	Coverage Δ
src/subprocess.h	62.5% <0%> (-8.34%)	:arrow_down:
src/sub-linux.cc	67.2% <0%> (-3.21%)	:arrow_down:

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