On Windows, os.Chmod and syscall.Chmod toggle the FILE_ATTRIBUTES_
READONLY flag depending on the permission bits. That's a bit odd but I
guess some compromises were made at some point and this is what was
chosen to map to a Unix concept that Windows doesn't really have in the
same way. That's fine. However, the logic used in Chmod was forgotten
from os.Open and syscall.Open, which then manifested itself in various
places, most recently, go modules' read-only behavior.

This makes syscall.Open consistent with syscall.Chmod and adds a test
for the permission _behavior_ using ioutil. By testing the behavior
instead of explicitly testing for the attribute bits we care about, we
make sure this doesn't regress in unforeseen ways in the future, as well
as ensuring the test works on platforms other than Windows.

In the process, we fix some tests that never worked and relied on broken
behavior, as well as tests that were disabled on Windows due to the
broken behavior and had TODO notes.

Fixes #35033

Change-Id: I6f7cf54517cbe5f6b1678d1c24f2ab337edcc7f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/202439
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>