test-includes: improve portability

FreeBSD's shell permits [^bar] to match characters not in [bar] like
common regular expressions. This is non-portable and '!' should be
used in place of '^' for portability.

Reported by: Nathaniel Wesley Filardo
Issue: https://github.com/CTSRD-CHERI/cheribsd/issues/1321
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D34597