init.sh: accommodate even those who specify bogus srcdir manually

* tests/init.sh: Normally, srcdir is guaranteed by automake and
configure-time tests to be sanitized, so that there is no need to
use "$srcdir" in Makefile rules and shell scripts.  Using $srcdir
(with no double quotes) suffices.  However, since tests may be
invoked manually, and since you may explicitly set srcdir to the
name of a directory containing spaces, do quote its uses here.
* tests/test-pread.sh: Likewise.
Suggested by Bruno Haible.