Add regression tests for duplicated needle and fix css needle.

[mshima]

• bug-fix: dropped pipeline-pipe due to not handling errors correctly.
• bug-fix: when adding styles, ignore comments when testing if it's already included.
• test added: when adding same needles twice, they should not be added and files should not change.
• misc improvements to needle tests.

Closes #14388.

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[pascalgrimaud]

@mshima : are you confident enough, for merging this ?
The release is for tomorrow and I think we need this bug fix

[mshima]

@pascalgrimaud yes.
The first change is important due to prettier error handling. IMO It’s well tested (every test tests it 😄).
The second is a very small bug-fix.

Merging.

[pascalgrimaud]

Thanks a lot, don't forget your bounty