t/*: rearrange tests into numbered directories.

It was getting way too ad-hoc in there.  Let's reorganize the tests so that
there's a good, obvious, suggested sequence to run them in.

t/101-atime/atime2.do

@@ -0,0 +1,6 @@
+# make sure redo doesn't think merely *reading* the old file counts as
+# modifying it in-place.
+cat $1 >/dev/null 2>/dev/null || true
+./tick
+cat $1 >/dev/null 2>/dev/null || true
+echo hello