Implement a domtree sub-test.

This test verifies the computed dominator tree against annotations.

Move the existing testcases into filetests/ with the new syntax.

filetests/domtree/loops.cton

@@ -0,0 +1,20 @@
+test domtree
+
+function test(i32) {
+    ebb0(v0: i32):
+        brz v0, ebb1  ; dominates: ebb1 ebb3 ebb4 ebb5
+        jump ebb2     ; dominates: ebb2
+    ebb1:
+        jump ebb3
+    ebb2:
+        brz v0, ebb4
+        jump ebb5
+    ebb3:
+        jump ebb4
+    ebb4:
+        brz v0, ebb3
+        jump ebb5
+    ebb5:
+        brz v0, ebb4
+        return
+}