cmake: allow building tests in unity mode

cmake: allow building tests in unity mode

Makes building tests noticeably faster.

Apply changes/fixes/workarounds to make Unity work:
- rename test variables to avoid collisions or shadowing each other when
  combined into single units.
- add workaround to avoid applying `lib/memdebug.h` overrides to system
  headers declaring/defining `getaddrinfo()`/`freeaddrinfo()` for
  `tests/server/resolve.c`. This replaces a previous workaround that
  worked for that specific source.
- rename test macro `CTRL` clashing with Cygwin `sys/ioctl.h`.
- add include guard to `test.h`.

Also:
- exclude `tests/http/clients` which are all single-source. (like
  `docs/examples`.)

Build time improvements for tests:
- AppVeyor CI:
  - MSVC 2008, 2010: 1 minute faster (4m8s -> 2m56s, 3m19s -> 2m24s)
  - MSVC 2022 arm64: 3.5 minutes faster (10m18s -> 6m48s)
  before: https://ci.appveyor.com/project/curlorg/curl/builds/50522785
  after: https://ci.appveyor.com/project/curlorg/curl/builds/50522942
- GHA:
  - Cygwin: 1.5 minutes faster (3m13s -> 1m43s)
    before: https://github.com/curl/curl/actions/runs/10681535327/job/29605384398
    after: https://github.com/curl/curl/actions/runs/10680818726/job/29603130637
  - Windows:
    before: https://github.com/curl/curl/actions/runs/10680818713
    after: https://github.com/curl/curl/actions/runs/10683850187
    - MSYS2, mingw-w64: 1 minute faster
    - MSVC: 30 seconds faster (3m17s -> 2m48s)
  - macOS: double speed (39s -> 18s)
    before: https://github.com/curl/curl/actions/runs/10680818753/job/29603133447
    after: https://github.com/curl/curl/actions/runs/10683850174/job/29612914515
  - Linux: almost double speed (30/31s -> 18s)
    before: https://github.com/curl/curl/actions/runs/10681535311/job/29605387156
    after: https://github.com/curl/curl/actions/runs/10680818721/job/29603133976
  - non-native: no obvious effect.
    before: https://github.com/curl/curl/actions/runs/10680818722
    after: https://github.com/curl/curl/actions/runs/10683850187
  - Old Linux: Unity mode not supported by old CMake, no effect.

Closes #14765