Fix test for command success.
Exit 99 when no tests exist in specified directory.

Certainly, no tests existing should not exit without an error. The
reason for choosing 99 is that automake treats that as a "hard" error.
Remove unnecessary quoting.

Since apparently quotes don't care about newlines in Ruby, it's much
clearer to not have the newline escapes and concatenated strings.
Make spec directory into a non-switch argument.