ARROW-3000: [C++] Add option to label test groups then only build those unit tests

[wesm]

This should help make our groups of unit tests more modular and easier to build only the relevant ones for a particular test cycle

[wesm]

This isn't quite working yet, I'll take a look:

0.16s$ $TRAVIS_BUILD_DIR/ci/travis_script_cpp.sh
~/build/apache/arrow/cpp-build ~/build/apache/arrow
Test project /home/travis/build/apache/arrow/cpp-build
No tests were found!!!
~/build/apache/arrow
~/build/apache/arrow ~/build/apache/arrow
geninfo: ERROR: no .gcda files found in .!
~/build/apache/arrow

[wesm]

The C++/Python build ran in 31 minutes this time

[codecov-io]

Codecov Report

Merging #2406 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2406      +/-   ##
==========================================
+ Coverage   84.67%   84.73%   +0.06%     
==========================================
  Files         293      293              
  Lines       45459    45474      +15     
==========================================
+ Hits        38492    38533      +41     
+ Misses       6926     6904      -22     
+ Partials       41       37       -4

Impacted Files	Coverage Δ
go/arrow/math/uint64_sse4_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/int64_sse4_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/float64_sse4_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/memory/memory_sse4_amd64.go	0% <0%> (-100%)	⬇️
python/pyarrow/plasma.py	87.5% <0%> (-0.22%)	⬇️
go/arrow/math/float64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/int64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/uint64_amd64.go	33.33% <0%> (ø)	⬆️
cpp/src/plasma/store.cc	89.64% <0%> (+0.42%)	⬆️
go/arrow/math/math_amd64.go	36.84% <0%> (+5.26%)	⬆️
... and 6 more

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[wesm]

+1