Refactor lib/File/stat.t for robustness.

Refactor lib/File/stat.t for robustness.

Use CORE::stat instead of stat, to be clear which is the builtin, and which
is the routine that File::stat is prepared to export. As it is now
unambiguous which is which, remove comments that annotated each use.

Use isa_ok() in place of ok() to test the return value of File::stat::stat.
Use is_deeply() in place of is() and arrays interpolated into strings.

Move the data driven loop that tests most of the -X operators into a
function test_X_ops(), and use this to test both the tempfile
(non-executable) and $^X (executable).

Put the first sanity test of File::stat::stat inside a block so that its
lexicals don't leak.