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>