Add gas test confirming that br instructions do not end blocks.

tests/expectations/gas/branch.wat

@@ -0,0 +1,29 @@
+(module
+  (type (;0;) (func (result i32)))
+  (type (;1;) (func (param i32)))
+  (import "env" "gas" (func (;0;) (type 1)))
+  (func (;1;) (type 0) (result i32)
+    (local i32 i32)
+    i32.const 3
+    call 0
+    block  ;; label = @1
+      i32.const 17
+      call 0
+      i32.const 0
+      set_local 0
+      i32.const 1
+      set_local 1
+      get_local 0
+      get_local 1
+      tee_local 0
+      i32.add
+      set_local 1
+      i32.const 1
+      br_if 0 (;@1;)
+      get_local 0
+      get_local 1
+      tee_local 0
+      i32.add
+      set_local 1
+    end
+    get_local 1))