test_time: rewrite PyTime API rounding tests

Drop all hardcoded tests. Instead, reimplement each function in Python, usually
using decimal.Decimal for the rounding mode.

Add much more values to the dataset. Test various timestamp units from
picroseconds to seconds, in integer and float.

Enhance also _PyTime_AsSecondsDouble().

Python/pytime.c

@@ -324,12 +324,16 @@ _PyTime_FromMillisecondsObject(_PyTime_t *t, PyObject *obj, _PyTime_round_t roun
 double
 _PyTime_AsSecondsDouble(_PyTime_t t)
 {
-    _PyTime_t sec, ns;
-    /* Divide using integers to avoid rounding issues on the integer part.
-       1e-9 cannot be stored exactly in IEEE 64-bit. */
-    sec = t / SEC_TO_NS;
-    ns = t % SEC_TO_NS;
-    return (double)sec + (double)ns * 1e-9;
+    if (t % SEC_TO_NS == 0) {
+        _PyTime_t secs;
+        /* Divide using integers to avoid rounding issues on the integer part.
+           1e-9 cannot be stored exactly in IEEE 64-bit. */
+        secs = t / SEC_TO_NS;
+        return (double)secs;
+    }
+    else {
+        return (double)t / 1e9;
+    }
 }
 
 PyObject *