Add unit tests for WASM related code; 为wasm模块补充单测

[seeflood]

What would you like to be added:
Add unit tests for WASM related code:
understand the implementation of the wasm module and add unit tests for it. The unit test coverage of the module should reach 60%.
If you encounter any problems in the process, you can ask @zhenjunMa for help.
The code path is pkg/wasm/

By doing this improvement,you will learn that how Layotto supports multilingual programming based on WASM

chinese:
看明白wasm模块的实现原理并补充单测，使模块的单测覆盖率达到60%
过程中遇到问题可以问 @zhenjunMa
代码路径为 pkg/wasm/

做完这些优化后，你将学习到Layotto如何通过wasm支持多语言编程

Why is this needed:
Currently the test coverage of wasm is low:

[seeflood]

For your reference:
quickstart demo for using WASM
or
quickstart demo for using WASM(中文)

[oneyue610]

OK

[seeflood]

OK

Ok,already assigned.

[seeflood]

@oneyue610 Hi, are you still working on it?

[seeflood]

@oneyue610 Hi,are you still working on it?
Since you didn't response,I will reassign this task to someone else :<
If you still want to do this,just tell me and i will assign to u again

[Xunzhuo]

@assign