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/traps_early.cton

@@ -0,0 +1,20 @@
+; For testing cfg generation. This code explores the implications of encountering
+; a terminating instruction before any connections have been made.
+test print-cfg
+
+function nonsense(i32) {
+; check: digraph nonsense {
+
+ebb0(v1: i32):
+    trap
+    brnz v1, ebb2   ; unordered: ebb0:inst1 -> ebb2
+    jump ebb1       ; unordered: ebb0:inst2 -> ebb1
+
+ebb1:
+    v2 = iconst.i32 0
+    v3 = iadd v1, v3
+    jump ebb0(v3)   ; unordered: ebb1:inst5 -> ebb0
+
+ebb2:
+    return v1
+}