run-tests.sh hides the output from the compiler unless the compilation
fails. To recognize newly introduced warnings use -Werror by default.

Also add a switch to disable -Werror in case the warnings are expected.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240423-nolibc-werror-v1-1-e6f0bd66eb45@weissschuh.net