Fix tests on 1.8
csfml 2.0 (new formula)

CSFML are the C bindings to the SFML library (already present in
Homebrew).
sfml: add 2.1 bottle.
csfml: add 2.0 bottle.

Closes #31762.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
csfml: improve test.