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/loop.cton

@@ -0,0 +1,34 @@
+; For testing cfg generation. This code is nonsense.
+test print-cfg
+
+function nonsense(i32, i32) -> f32 {
+; check: digraph nonsense {
+; regex: I=\binst\d+\b
+; check: label="{ebb0 | <$(BRZ=$I)>brz ebb2 | <$(JUMP=$I)>jump ebb1}"]
+
+ebb0(v1: i32, v2: i32):
+    v3 = f64const 0x0.0
+    brz v2, ebb2            ; unordered: ebb0:$BRZ -> ebb2
+    v4 = iconst.i32 0
+    jump ebb1(v4)           ; unordered: ebb0:$JUMP -> ebb1
+
+ebb1(v5: i32):
+    v6 = imul_imm v5, 4
+    v7 = iadd v1, v6
+    v8 = f32const 0.0
+    v9 = f32const 0.0
+    v10 = f32const 0.0
+    v11 = fadd v9, v10
+    v12 = iadd_imm v5, 1
+    v13 = icmp ult, v12, v2
+    brnz v13, ebb1(v12)     ; unordered: ebb1:inst12 -> ebb1
+    v14 = f64const 0.0
+    v15 = f64const 0.0
+    v16 = fdiv v14, v15
+    v17 = f32const 0.0
+    return v17
+
+ebb2:
+    v100 = f32const 0.0
+    return v100
+}