Add tests for tables.clear()

This should reduce the chance of regressions.

tests/collections/ttables.nim

@@ -134,6 +134,27 @@ block mpairsTableTest1:
 block SyntaxTest:
   var x = toTable[int, string]({:})
 
+block clearTableTest:
+  var t = data.toTable
+  assert t.len() != 0
+  t.clear()
+  assert t.len() == 0
+
+block clearOrderedTableTest:
+  var t = data.toOrderedTable
+  assert t.len() != 0
+  t.clear()
+  assert t.len() == 0
+
+block clearCountTableTest:
+  var t = initCountTable[string]()
+  t.inc("90", 3)
+  t.inc("12", 2)
+  t.inc("34", 1)
+  assert t.len() != 0
+  t.clear()
+  assert t.len() == 0
+
 proc orderedTableSortTest() =
   var t = initOrderedTable[string, int](2)
   for key, val in items(data): t[key] = val