Add per-test KPI support

tests can now use 'tenv.test().set_kpis(some_dict)' to set any kind of
data as KPIs, which will be presented in the junit report.

The representation of KPIs in the xml file doesn't follow the junit
format, mainly because it has no support for per-test properties.

Change-Id: I00e976f65a202e82d440bf33708f06c8ce2643e2
diff --git a/selftest/report_test/report_test.ok b/selftest/report_test/report_test.ok
index 87092ea..442d343 100644
--- a/selftest/report_test/report_test.ok
+++ b/selftest/report_test/report_test.ok
@@ -15,3 +15,5 @@
 tst {combining_scenarios='config'}: DBG: {definition_conf={}}  [suiteC↪{combining_scenarios='config'}]
 tst suiteD: DBG: {combining='config'}
 tst {combining_scenarios='config'}: DBG: {definition_conf={}}  [suiteD↪{combining_scenarios='config'}]
+tst suiteE: DBG: {combining='config'}
+tst {combining_scenarios='config'}: DBG: {definition_conf={}}  [suiteE↪{combining_scenarios='config'}]