Refactor: libcrmcommon: Avoid snprintf() in pcmk__set_env_option()
To avoid the need to error-check snprintf() explicitly, which we have
not been doing.
Drop a couple of unit tests, since we're no longer using a fixed-size
buffer and returning an error when it's too small. Also use CPP
constants and variables where appropriate in the unit tests.
Ref T974
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>