unit: write out XML file

This writes out the results of the unit tests in an XML file called
lwip_unittests.xml in the same directory as the unittests executable.

See https://libcheck.github.io/check/doc/check_html/check_4.html#XML-Logging
for an example of the ouput. Of particular use is the duration field, for
each test and for the entire test run.

test/unit/lwip_unittests.c

@@ -86,6 +86,7 @@ int main(void)
 #endif
 
   sr = srunner_create((suites[0])());
+  srunner_set_xml(sr, "lwip_unittests.xml");
   for(i = 1; i < num; i++) {
     srunner_add_suite(sr, ((suite_getter_fn*)suites[i])());
   }