Enhance the srctop, bldtop, data and result functions to check the result

Enhance the srctop, bldtop, data and result functions to check the result

This affects bldtop_dir, bldtop_file, srctop_dir, srctop_file,
data_dir, data_file, result_dir, and result_file.  They are all
enhanced to check that the resulting path really is a directory or a
file.  They only do this if the path exists.

This allows the tests to catch if these functions are used
incorrectly, even on systems where the syntax for directories and
files is the same.

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16523)