X Fix race in test

_blkaccess was called after send to have_read, thus a user could be
woken up before the block was actually marked as accessed and so
there is a window while _pinnedAt will return wrong data due to having
stale accessed set.

The bug was showing itself as assert happening from time to time
seldomly on e.g. `assert w.pinned = pin_prev` in watch.

wcfs/wcfs_test.py

@@ -564,8 +564,8 @@ class tFile:
             have_read = chan(1)
             def _():
                 b = read_nogil(blkview[0:1])
-                have_read.send(b)
                 t._blkaccess(blk)
+                have_read.send(b)
             go(_)
             _, _rx = select(
                 ctx.done().recv,    # 0