Search in build/ sub-dir if only suite root is given

If the package suite was given (e.g. DUNE_ROOT=/blum), then the code
set up the root for each individual package automatically (e.g.
DUNE_COMMON_ROOT=/blum/dune-common), but the path which was then
activated did not get the local build sub-directory (e.g. if we are
building opm-autodiff in /frub/opm-autodiff/build, then the local
build directory is "build/"), and thus this was not appended to the
library search path. The result was that the source was found (because
the root pointed to a valid source tree), but the library was not
(because it is "hidden" in the subdirectory).
Only add build sub-directory if it is actually used

Otherwise the search will cascade down one level too many.
Only add parent directories for actual variants

Otherwise we'll inadvertedly add the root directory and will probably
find the system implementation underneath there.
runtest.sh: fix shellcheck warninings

at least the ones which were no false positives