Updated docs about HTTP testing

[CodeF0x]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The order of the http testing related providers is pretty important and doing it wrong made my tests fail.

What is the new behavior?

Added a small heads up to always keep the order in mind

Does this PR introduce a breaking change?

• Yes
• No

Other information

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[JeanMeche]

Hi, can you please sign the CLA for the PR to go foward, thank you.

[CodeF0x]

Hi @JeanMeche, sorry for keeping you waiting. I signed the CLA.

[JeanMeche]

Can you check that you signed it with <co****t​@codef0x.dev> (the email used by the commit), the check still returns an error.

[CodeF0x]

I triggered the CLA check again, now it seems to work.