Add basic tests for out-of-range epoch int to timestamp conversions (#500)

Author: l46kok

tests/simple/testdata/timestamps.textproto

@@ -380,13 +380,27 @@ section {
       errors { message: "range" }
     }
   }
+  test {
+    name: "from_int_under"
+    expr: "timestamp(-62135596801)"
+    eval_error {
+      errors { message: "range" }
+    }
+  }
   test {
     name: "from_string_over"
     expr: "timestamp('10000-01-01T00:00:00Z')"
     eval_error {
       errors { message: "range" }
     }
   }
+  test {
+    name: "from_int_over"
+    expr: "timestamp(253402300800)"
+    eval_error {
+      errors { message: "range" }
+    }
+  }
   test {
     name: "add_duration_under"
     expr: "timestamp('0001-01-01T00:00:00Z') + duration('-1s')"