[clang-tidy] improve readability-identifier-naming hungarian options test

[amurzeau]

Summary

Adjust identifier-naming-hungarian-notation-cfgfile test with
changes for all configs in .clang-tidy to ensure they are really taken
into account.

This is done this way:

• Set all HungarianNotation.* options to cust prefix + their default value.
• Set TreatStructAsClass to true as it defaults to false.
• Because of the TreatStructAsClass change, add a test to check that structs are really checked the same way as classes (see struct StructCase).

Also, TreatStructAsClass was wrongly named in the test's .clang-tidy file
and was fixed (it is in General, not in Options).

Depends on D144431

[carlosgalvezp]

LGTM!

[carlosgalvezp]

@amurzeau I'm having some trouble downloading the patch, would you mind trying out a rebase on top of latest master? Otherwise I'll try downloading the raw diff.

[amurzeau]

Rebase on main