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>