tests: skip schema validation tests if xmllint is missing

On IRC, someone complained that a system without xmllint installed
failed a number of tests.

* tests/schematestutils.sh: Probe for xmllint.
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/schematestutils.sh

 #!/bin/sh
 
+(xmllint --version) >/dev/null 2>&1 || skip_test_ 'Missing xmllint'
+
 check_schema () {
 
 DIRS=$1