Add gas tests.

tests/fixtures/gas/simple.wat

@@ -0,0 +1,15 @@
+(module
+	(func (export "simple")
+		(if (i32.const 1)
+			(loop
+				i32.const 123
+				drop
+			)
+		)
+	)
+
+	(func
+		block
+		end
+	)
+)

tests/fixtures/gas/start.wat

@@ -0,0 +1,18 @@
+(module
+	(import "env" "ext_return" (func $ext_return (param i32 i32)))
+	(import "env" "memory" (memory 1 1))
+
+	(start $start)
+	(func $start
+		(call $ext_return
+			(i32.const 8)
+			(i32.const 4)
+		)
+		(unreachable)
+	)
+
+	(func (export "call")
+	)
+
+	(data (i32.const 8) "\01\02\03\04")
+)