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/121-defaults-nested/a/b/default.y.z.do

@@ -0,0 +1 @@
+echo default.y.z $2 ${1#$2}

t/121-defaults-nested/a/b/file.x.y.z.do

@@ -0,0 +1 @@
+echo file $2 ${1#$2}