ExtUtils-Install/t/Install.t: better -w tests

I recently added some tests to Install.t to check that files weren't
writeable. I used "-w $file", which gives a false positive if run as
root. So this commit changes the test to do a stat then test (mode & 0200);
hopefully that emulation on non-POSIX platforms is good enough for this to
be a valid test. perlport makes no mention of what stat->mode contains
on non-POSIX platforms.

(Chris fixed this this with 84d7dacc0cf1f by skipping the test if running
as root, but the current commit is more robust, in that is still tests for
correctness under root.)