Using a mixture of techniques to avoid symbols collisions:
- reduce scope.
- add `t*_` / `T*_` prefix.
- move shared functions to `testutil.c`.
(`suburl()`, `rlim2str()`)
- clone re-used lib*.c sources.
(lib587, lib645)
- include shared symbols just once in re-used `lib*.c` sources.
(using `LIB*_C` guards.)
- drop re-used `lib*.c` sources where they were identical or
unused.
- make macros global.
- #undef macros before use.
What remain is the entry functions `test`, and `unit_setup`,
`unit_stop` in unit tests.
Also:
- fix formatting and other minor things along the way.
- add `const` where possible.
- sync some symbol names between tests.
- drop `mk-bundle-hints.sh` that's no longer necessary.
Closes#17468
