[C++][CI] Set CTEST_PARALLEL_LEVEL to $concurrency

[asfimport]

Currently the default test target runs serially while unittest arbitrarily uses 4 threads. On many systems that's suboptimal. The environment variable CTEST_PARALLEL_LEVEL can be set to run tests in parallel, and we should probably have a good default for it (like the hardware concurrency)

https://cmake.org/cmake/help/latest/manual/ctest.1.html

Reporter: Ben Kietzman / @bkietz

_{Note: This issue was originally created as ARROW-8012. Please see the migration documentation for further details.}