tests: add tests for optimization mangleExports

[anshumanv]

What kind of change does this PR introduce?

tests

Did you add tests for your changes?
Yes

If relevant, did you update the documentation?
NA

Summary
A case was reported by the user in #1677 add tests for it

Does this PR introduce a breaking change?
No

Other information

fixes #1677

[snitin315]

@anshumanv with a new release (v5.0.0-beta.23) now webpack exports --optimization-mangle-exports, so it will be automatically tested inside coreFlags/optimization-flags.test.js. So we need to modify it there.
This new addition is the reason for CI failure (and some deprecation warnings).
Can you fix it?

[anshumanv]

This new addition is the reason for CI failure (and some deprecation warnings).

A new warning popped up in various tests, all need update

[anshumanv]

/cc @webpack/cli-team several tests are failing because the next version of webpack is throwing a warning so all the expect(stderr).toBeFalsy() assertions are failing, what do you think is the most suitable fix for this?

[snitin315]

I think we need to investigate where is the warning coming from and fix it. The waring is something like

(node:4632) [DEP_WEBPACK_COMPILATION_CACHE] DeprecationWarning: Compilation.cache was removed in favor of Compilation.getCache()
    (Use node --trace-deprecation ... to show where the warning was created)

[anshumanv]

I think we need to investigate where is the warning coming from and fix it. The waring is something like

Feel free to investigate and PR @snitin315

[snitin315]

@anshumanv the warning is coming from terser-webpack-plugin we need to PR there. I will send it.

https://github.com/webpack-contrib/terser-webpack-plugin/blob/91642a150ee2c72d0d445a852410b8ecf647a5f0/src/Webpack5Cache.js#L13-L15

[snitin315]

Now we wait for merge and release /cc @evilebottnawi

[anshumanv]

@anshumanv the warning is coming from terser-webpack-plugin we need to PR there. I will send it.

Awesome, thanks for the effort! 👏

[webpack-bot]

@anshumanv Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@evilebottnawi Please review the new changes.