Fix statistics system test on Oracle Linux 7

Fix statistics system test on Oracle Linux 7

The statistics system test fails on Oracle Linux 7 when libxml2, Curl,
and xsltproc are present:

    I:statistics:checking bind9.xsl vs xml (17)
    diff: curl.out.17.xsl: No such file or directory
    tests.sh: line 183: curl.out.17.xml: No such file or directory
    cp: cannot stat 'curl.out.17.xml': No such file or directory
    grep: xsltproc.out.17: No such file or directory

This is because the Oracle Linux 7 Curl does not know about the
--http1.1 option and silently fails with:

    + /usr/bin/curl --http1.1 http://10.53.0.3:7252
    curl: option --http1.1: is unknown
    curl: try 'curl --help' or 'curl --manual' for more information

The following test "checking bind9.xml socket statistics" then needs to
check for existence of stats.xml.out file which is artifact of the
previous test.