tests: loader's tests for errors and warnings

[snitin315]

What kind of change does this PR introduce?
test

Did you add tests for your changes?
Yes
If relevant, did you update the documentation?
NA
Summary
Fixes #1581

Does this PR introduce a breaking change?
No

Other information

[alexander-akait]

deprecations from ts-loader?

[snitin315]

Yes, can we test on stdout only?

[alexander-akait]

Yes, let comment stderr (with comment on issue), open an issue in ts-loader and merge it

[alexander-akait]

Need improve test

[webpack-bot]

@snitin315 Thanks for your update.

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

@evilebottnawi Please review the new changes.

[alexander-akait]

Hope it was fixed after removing execa

[snitin315]

still same 😞

[alexander-akait]

@snitin315 We need to solve it before release, there are two possible problems:

• our test utils is not respect exit code
• something wrong inside webpack-cli logic in try/catch

[alexander-akait]

/cc @snitin315 Can we focus on it, because it is high priority for new release, we don't want break linter-loaders and other stuff what potential change exit code

[snitin315]

Yes, I'm looking into it.

[alexander-akait]

We should refactor https://github.com/webpack/webpack-cli/blob/next/packages/webpack-cli/lib/utils/Compiler.js 😞 It is "kitchen sink"

[alexander-akait]

One note

[alexander-akait]

/cc @webpack/cli-team we need fix it before release