integration: test2json: enable timestamps to fix zero-time test durations

Without these options set, test2json does not include a `Time`
field in the generated JSON;

    {"Action":"run","Test":"TestCgroupNamespacesBuild"}
    {"Action":"output","Test":"TestCgroupNamespacesBuild","Output":"=== RUN   TestCgroupNamespacesBuild\n"}
    {"Action":"output","Test":"TestCgroupNamespacesBuild","Output":"--- PASS: TestCgroupNamespacesBuild (1.70s)\n"}
    ...
    {"Action":"pass","Test":"TestCgroupNamespacesBuild"}

As a result, `gotestsum` was not able to calculate test-duration, and
reported `time="0.000000"` for all tests;

    <testcase classname="amd64.integration.build" name="TestCgroupNamespacesBuild" time="0.000000"></testcase>

With this patch applied:

    {"Time":"2019-08-23T22:42:41.644361357Z","Action":"run","Package":"amd64.integration.build","Test":"TestCgroupNamespacesBuild"}
    {"Time":"2019-08-23T22:42:41.644367647Z","Action":"output","Package":"amd64.integration.build","Test":"TestCgroupNamespacesBuild","Output":"=== RUN   TestCgroupNamespacesBuild\n"}
    {"Time":"2019-08-23T22:42:44.926933252Z","Action":"output","Package":"amd64.integration.build","Test":"TestCgroupNamespacesBuild","Output":"--- PASS: TestCgroupNamespacesBuild (3.28s)\n"}
    ...
    {"Time":"2019-08-23T22:42:44.927003836Z","Action":"pass","Package":"amd64.integration.build","Test":"TestCgroupNamespacesBuild","Elapsed":3.28}

Which now correctly reports the test's duration:

    <testcase classname="amd64.integration.build" name="TestCgroupNamespacesBuild" time="3.280000"></testcase>

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/make/.integration-test-helpers

@@ -67,7 +67,7 @@ run_test_integration_suites() {
 				--jsonfile="${ABS_DEST}/$(basename "$dir")-go-test-report.json" \
 				--junitfile="${ABS_DEST}/$(basename "$dir")-junit-report.xml" \
 				--raw-command \
-				-- go tool test2json ./test.main ${flags}
+				-- go tool test2json -t ./test.main ${flags}
 		); then exit 1; fi
 	done
 }