Fixed race conditions

assert.sh fail in a couple cases

1) using assert, if your tested command has as parameter a string with a space separated asterisk then the asterisk will be evaluated by the shell.

2) using assert, if your tested command has as parameter a string with characters that have a special meaning in printf, printf will output something unexpected

I suspect that in assert_raises
(eval $1 <<< ${3:-}) > /dev/null 2>&1
should really be written as
(eval "$1" <<< ${3:-}) > /dev/null 2>&1

but I can't produce a test case similar to 1)

p.s.
I couldn't figure out the meaning of "x required to overwrite older results": is it still needed once printf is gone?