REGTEST: add the option to test only a specific set of files

REGTEST: add the option to test only a specific set of files

It currently is quite difficult to re-reun a specific test after an
error occurs. This patch adds a REG_TEST_FILES variable to the makefile,
which will be used to override the find operation. This helps focusing
on a specific file, which is essential during bisect to figure what
commit introduced a specific regression. Multiple files may be tested,
the return code will indicate the number of failed tests.