[HttpFoundation] Improve string values in test codes
minor #11812 [HttpFoundation] Improve string values in test codes (DQNEO)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11812).

Discussion
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[HttpFoundation] Improve string values in test codes

I made the meanings of string literals more clear.

| Q             | A
| ------------- | ---
| Bug fix?      | no,
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
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05f5860 [HttpFoundation] Improve string values in test codes