tests/lapi: fix assertion with result in os_time_test

The `mktime()` function (which is used inside `os.time()`)
returns -1 for cases when the time since the Epoch cannot be
represented. In this case, `os.time()` returns `nil`:

```
luajit -e "print(os.time({year = 1970, month = 1, day = 1, sec = -9 * 60 * 60 - 1}))"
nil
```

The patch fixes an assertion in the test and allows `nil` as
a result of the function.

Follows up #141

Author: ligurio

tests/lapi/os_time_test.lua

@@ -32,7 +32,11 @@ local function TestOneInput(buf)
     local err_handler = test_lib.err_handler(ignored_msgs)
     local ok, res = xpcall(os.time, err_handler, time)
     if not ok then return end
-    assert(type(res) == "number" or type(res) == "table")
+    io.stderr:write(type(res) .. "\n")
+    assert(type(res) == "number" or
+           type(res) == "table" or
+           -- Undocumented.
+           res == nil)
 end
 
 local args = {