tests: add unit tests for ClockObject

Closes #593

tests/putil/conftest.py

@@ -0,0 +1,6 @@
+import pytest
+from panda3d.core import ClockObject
+
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+def clockobj():
+    return ClockObject()

tests/putil/test_clockobject.py

@@ -0,0 +1,23 @@
+import time
+
+def test_get_frame_time(clockobj):
+    current_time = clockobj.get_frame_time()
+    time.sleep(2)
+    assert clockobj.get_frame_time() == current_time
+
+def test_jump_frame_time(clockobj):
+    current_time = clockobj.get_frame_time()
+    clockobj.tick()
+    assert clockobj.get_frame_time() == current_time + clockobj.get_frame_time()
+
+def test_get_real_time(clockobj):
+    current_time = clockobj.get_real_time()
+    time.sleep(2)
+    assert current_time != clockobj.get_real_time()
+
+def test_get_dt(clockobj):
+    clockobj.tick()
+    first_tick = clockobj.get_frame_time()
+    clockobj.tick()
+    second_tick = clockobj.get_frame_time()
+    assert clockobj.get_dt() == second_tick - first_tick