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

tests/fixtures/gas/branch.wat

@@ -0,0 +1,27 @@
+(module
+	(func $fibonacci_with_break (result i32)
+		(local $x i32) (local $y i32)
+
+		(block $unrolled_loop
+			(set_local $x (i32.const 0))
+			(set_local $y (i32.const 1))
+
+			get_local $x
+			get_local $y
+			tee_local $x
+			i32.add
+			set_local $y
+
+			i32.const 1
+			br_if $unrolled_loop
+
+			get_local $x
+			get_local $y
+			tee_local $x
+			i32.add
+			set_local $y
+		)
+
+		get_local $y
+	)
+)