selftests/nolibc: make evaluation of test conditions

If "cond" is a multi-token statement the behavior of the preprocessor
will lead to the negation "!" to be only applied to the first token.
Although currently no test uses such multi-token conditions but it can
happen at any time.

Put braces around "cond" to ensure the negation works as expected.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>