Implement support for information_schema.files in InnoDB so that it can
be a replacement for i_s.innodb_sys_datafiles. Add similar SELECT
statements for i_s.files wherever i_s.innodb_sys_datafiles is used in
all test files.  Gather information from the fil_system in fil0fil.cc
one file at a time so that the fil_system->mutex is not held very long.

Approved by Marko and Vasil in rb#7396