Proposal to move the benchmark executables into a seperate build subdir

[Anja <an...@gmail.com>]

Hello!

The benchmark executables are placed in the same directory as the other
test executables:
https://github.com/apache/arrow/blob/b4ac585ecb4da610cc64e346e564ca86594aec53/cpp/cmake_modules/BuildUtils.cmake#L614.


This means that if somebody builds the benchmarks with
`ARROW_BUILD_BENCHMARK=ON`,
and then naively runs `ctest .` in the test folder, they will also be
running the very slow benchmark tests.

(There does exist a workaround, which is to use the flag `--label-regex
unittest`.)

Considering that `ctest` is not the standard way the benchmark executables
are run, they are usually run with the archery utility (
https://arrow.apache.org/docs/developers/benchmarks.html), would there be
support in moving the benchmark executables to a seperate build
subdirectory from the unittest executables? Is there value in having them
in the same location, all runnable with `ctest .`?

~* Anja