testmain: add xml-wrap-results option

When --xml option is used, allow the --xml-wrap-results
option specify an additional test suite path to prepend/wrap
the test results output.

Normally, the xml output is like:

<TestLog>
 <TestSuite name="Wayland Functional Integration Test Suite">
  <TestSuite name="core">
  ...

With --xml-wrap-results specified, say as "my/custom/path", then
the xml output would look like:

<TestLog>
 <TestSuite name="Wayland Functional Integration Test Suite">
  <TestSuite name="my">
   <TestSuite name="custom">
    <TestSuite name="path">
     <TestSuite name="core">
     ...

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>