Tests: Respect TEST_TMPDIR, TMPDIR, TMP environmental variables

[Arfrever]

Use GetExistingTempDirectories() (which uses GetTempDirectories() which respects
TEST_TMPDIR, TMPDIR, TMP environmental variables) for FLAGS_test_tmpdir variable
used in some tests.

Fixes: #793

[codecov-commenter]

Codecov Report

Merging #794 (6d3f70a) into master (b38ed9d) will increase coverage by 0.24%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #794      +/-   ##
==========================================
+ Coverage   72.78%   73.02%   +0.24%     
==========================================
  Files          17       17              
  Lines        3259     3259              
==========================================
+ Hits         2372     2380       +8     
+ Misses        887      879       -8     

Impacted Files	Coverage Δ
src/glog/logging.h.in	80.00% <0.00%> (ø)
src/logging.cc	74.29% <0.00%> (+0.59%)	⬆️

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[sergiud]

Thanks for the PR. Only couple of suggestions.

[sergiud]

Thank you!