add exceptions check to is_testable_{,_xml}_file
add exceptions check to is_testable_{,_xml}_file

With the current code, whenever one of is_testable_file and
is_testable_xml_file is called, file_exceptions are removed from the
results before they are used (in one case after having been passed to
another function).

Adding the check for exceptions to is_testable_{,_xml}_file makes
the code of their callers more readable and eliminates one argument
from validate_individual_files (which still has 7 arguments left).

Change-Id: Ifdb38cb1a41d7a660eace505f7840e94c270fd8d

savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,openstack/openstack-doc-tools,openstack/openstack-doc-tools