Implement a domtree sub-test.

This test verifies the computed dominator tree against annotations.

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

cranelift/filetests/domtree/loops2.cton

@@ -0,0 +1,31 @@
+test domtree
+
+function test(i32) {
+    ebb0(v0: i32):
+        brz v0, ebb1    ; dominates: ebb1 ebb6
+        brnz v0, ebb2   ; dominates: ebb2 ebb9
+        jump ebb3       ; dominates: ebb3
+    ebb1:
+        jump ebb6
+    ebb2:
+        brz v0, ebb4    ; dominates: ebb4 ebb7 ebb8
+        jump ebb5       ; dominates: ebb5
+    ebb3:
+        jump ebb9
+    ebb4:
+        brz v0, ebb4
+        brnz v0, ebb6
+        jump ebb7
+    ebb5:
+        brz v0, ebb7
+        brnz v0, ebb8
+        jump ebb9
+    ebb6:
+        return
+    ebb7:
+        jump ebb8
+    ebb8:
+        return
+    ebb9:
+        return
+}