For rationale, see https://github.com/php/php-src/pull/6787
Make extension checks lowercase, add a special case for opcache
that has internal name not matching .so filename.
Extensions migrated in part 2:
* dom
* exif
* fileinfo
* ffi
tests for error1: not well formed xml
tests for error2: not well formed xml attribute without colon
tests for error2: not well formed xml, typo
tests for error4: wrong xml version
tests for error5: extra content at the end of the doc
tests libxml_dtd_load
tests libxml_dtd_valid
tests for libdtd_noerror
tests for LIBXML_DTDATTR, LIBXML_NOCDATA, LIBXML_NOENT and LIBXML_NOBLANKS
the basic behavior
fix comments