test-lib: clarify and tighten SANITY

test-lib: clarify and tighten SANITY

f400e51c (test-lib.sh: set prerequisite SANITY by testing what we
really need, 2015-01-27) improved the way SANITY prerequisite was
determined, but made the resulting code (incorrectly) imply that
SANITY is all about effects of permission bits of the containing
directory has on the files contained in it by the comment it added,
its log message and the actual tests.

State what SANITY is about more clearly in the comment, and test
that a file whose permission bits says should be unreadble truly
cannot be read.

Signed-off-by: Junio C Hamano <gitster@pobox.com>