Implement the 'test print-cfg' sub-test.

Move the CFG tests into the filetests directory.

Remove the tests directory, there are no more shell-driven tests left.

filetests/cfg/unused_node.cton

@@ -0,0 +1,21 @@
+; For testing cfg generation where some block is never reached.
+test print-cfg
+
+function not_reached(i32) -> i32 {
+; check: digraph not_reached {
+; check:     ebb0 [shape=record, label="{ebb0 | <inst0>brnz ebb2}"]
+; check:     ebb1 [shape=record, label="{ebb1 | <inst4>jump ebb0}"]
+; check:     ebb2 [shape=record, label="{ebb2}"]
+
+ebb0(v0: i32):
+    brnz v0, ebb2       ; unordered: ebb0:inst0 -> ebb2
+    trap
+
+ebb1:
+    v1 = iconst.i32 1
+    v2 = iadd v0, v1
+    jump ebb0(v2)       ; unordered: ebb1:inst4 -> ebb0
+
+ebb2:
+    return v0
+}