tests: use portable shell code

tests: use portable shell code

'make check' fails since commit 470d5c46 on any system with dash
as /bin/sh, because '<<<' is a bash extension.  For example:

nwfilterschematest: 23: /home/eblake/libvirt/tests/schematestutils.sh: Syntax error: redirection unexpected

Also, there is no need to spawn a grep process when shell globbing
can do the same.

* tests/schematestutils.sh: Replace bashism and subprocess with a
faster and portable construct.

Signed-off-by: Eric Blake <eblake@redhat.com>