[robotpy] Add wrapper for timestamp functions, like SetNowImpl (#8889)

`SetNowImpl` is used somewhat often in unit tests. It is a little bit
goofier to wrap because it takes a C function, so a little bit more work
has to be done to get that wrapped in pybind.

Claude helped.

wpiutil/src/test/python/test_timestamp.py

@@ -0,0 +1,46 @@
+
+import wpiutil
+import time
+import pytest
+
+def test_default():
+    wpi_now = wpiutil.now() * 1e-6
+    py_now = int(time.time())
+
+    # Allow a one second delta. We don't care about it being all that accurate in the
+    # test, just that we are in the same galaxy
+    assert py_now == pytest.approx(wpi_now, abs=1)
+
+
+NOW_TIMESTAMP_S = 0
+
+def custom_now_getter():
+    global NOW_TIMESTAMP_S
+    return int(NOW_TIMESTAMP_S * 1e6)
+
+
+@pytest.fixture
+def custom_fixture():
+    wpiutil.SetNowImpl(custom_now_getter)
+    yield
+    wpiutil.SetNowImpl(None)
+
+
+def test_custom_timestamp(custom_fixture):
+    global NOW_TIMESTAMP_S
+
+    assert 0 == wpiutil.now()
+
+    NOW_TIMESTAMP_S = 1.5
+    assert 1_500_000 == wpiutil.now()
+    
+    NOW_TIMESTAMP_S = 100
+    assert 100_000_000 == wpiutil.now()
+
+    # Set it back to the standard implementation and expect its roughly milliseconds since 1970
+    wpiutil.SetNowImpl(None)
+    wpi_now = wpiutil.now() * 1e-6
+    py_now = int(time.time())
+    assert py_now == pytest.approx(wpi_now, abs=1)
+
+