Simplify lib/File/stat.t by using a tempfile as the test victim.

Simplify lib/File/stat.t by using a tempfile as the test victim.

The previous code had got very gnarly trying to use a file from the
distribution for the test file, attempting to cope with
1) Other programs reading the file and hence the atime updating
2) The perl interpreter reading the file and hence the atime updating :-)
3) Building with -Dmksymlinks meaning that the file is a symlink

All these problems and work arounds simply *vanish* if we use a tempfile.
This will also enable us to change its mode in the future for better testing.