rename is_xml{,like}: is_testable{,_xml}_file

"is_xml_like" is a misnomer, because JSON files (or rather, files whose
names ends with .json) pass whereas pom.xml doesn't.

This patch renames is_xml to the more descriptive is_testable_xml_file
and is_xml_like to is_testable_file.

Change-Id: I4e2d1cbc0bec83e853c93823498bd356fe049748