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/350-deps/overwrite3.do

@@ -0,0 +1,8 @@
+# we don't delete $3 here, we just truncate and overwrite it.  But redo
+# can detect this by checking the current file position of our stdout when
+# we exit, and making sure it equals either 0 or the file size.
+#
+# If it doesn't, then we accidentally wrote to *both* stdout and a separate
+# file, and we should get warned about it.
+echo hello world
+echo goodbye world >$3