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/550-chdir/all.do

@@ -0,0 +1,18 @@
+. ../skip-if-minimal-do.sh
+
+rm -f chdir1
+redo chdir2
+redo chdir3
+
+../flush-cache
+redo-ifchange chdir3
+
+rm -f chdir1
+../flush-cache
+redo-ifchange chdir3
+[ -e chdir1 ] || exit 77
+
+rm -f chdir1
+../flush-cache
+redo-ifchange chdir3
+[ -e chdir1 ] || exit 78