Avoid Clang flooding with non-disableable warnings like:
T does not declare any constructor to initialize its non-modifiable members
when compiling with concepts check enabled.
Bug 41759: `warn_no_constructor_for_refconst` can not be disabled (Boost.ConceptCheck)
https://bugs.llvm.org/show_bug.cgi?id=41759
Split header tests accordingly i.e. test core headers as part of
core tests, numeric extension headers as part of numeric tests, etc.
It extends the convention of sub-directories already established in
`include/boost/gil` directory. It is sensible to follow it in other
areas of the source tree (i.e. `test/`, `doc/` and `benchmark/`).
Another important reason to move the tests is to enable removal of
the top-level `Jamfile` with all its definitions of test-specific
requirements.
The top-level `Jamfile` is not advised, especially if it specifies
build requirements like C++ language version.
Those affect non-tests builds e.g. documentation, causing failures
during generation of HTML documentation (leads to missing docs).
