To simplify dependencies, and sync tunits and units builds further.
`curlcheck.h` already depended on logic implemented within libtests:
it referenced a global variable (`unitfail`) defined in `first.c` and
declared in `test.h`.
Also:
- rename to `unitcheck.h` to indicate it's meant for unit tests.
- make `unitcheck.h` include `first.h` instead of `test.h`.
This brings header use closer to libtests. It also includes
`curlx/curlx.h` for all unit tests by default now.
- move `unitfail` declaration from `test.h` to `first.h`.
To match its definition in `first.c`.
- drop now redundant per-test curlx header includes.
Closes#17868
This test makes sure that a number of internal and public structs are
within their maximum allowed size limits.
The public structs can only grow in controlled ways, while the internal
ones may be allowed to grow if deemed right.
The idea here is to control, to know and make sure all important struct
growth is intentional.
Closes#17823
