keactrl tests uses not POSIX compliant '+='

The shell construct var+="<to-append>" construct is not POSIX compliant and should be replaced by <var>="${<var>}<to-append>" which BTW is very common in configure scripts...

This makes keactrl tests to not pass on NetBSD 8.0 and likely other BSDs at the exception of macOS where /bin/sh is in fact bash.