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/basic.cton

@@ -0,0 +1,13 @@
+test domtree
+
+function test(i32) {
+    ebb0(v0: i32):
+        jump ebb1     ; dominates: ebb1
+    ebb1:
+        brz v0, ebb3  ; dominates: ebb3
+        jump ebb2     ; dominates: ebb2
+    ebb2:
+        jump ebb3
+    ebb3:
+        return
+}