virtestmock: Print invalid file accesses into a file

virtestmock: Print invalid file accesses into a file

All the accesses to files outside our build or source directories
are now identified and appended into a file for later processing.
The location of the file that contains all the records can be
controlled via VIR_TEST_FILE_ACCESS env variable and defaults to
abs_builddir "/test_file_access.txt".

The script that will process the access file is to be added in
next commit.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>