qemu.tests: add timedrift_check_clock_offset cfg

Signed-off-by: NXu Tian <xutian@redhat.com>

qemu/tests/cfg/timedrift_check_clock_offset.cfg

+- check_clock_offset:
+    rtc_clock = host
+    rtc_base = utc
+    rtc_drift = slew
+    requires_root = yes
+    variants:
+        - with_syscall:
+            type = timedrift_check_with_syscall
+            start_vm = yes
+            only Linux
+            no JeOS
+            tmp_dir = "/tmp"
+            result_file = "clktest.log"
+            build_cmd = "gcc -lrt ${tmp_dir}/clktest.c -o clktest"
+            test_cmd = "./clktest"
+            check_timeout = 300