[v9_18] 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.

Backport of !6394 (merged)