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
