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

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

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D144510