Fix out-of-bound indices passed to `table.remove()`.

The patch fixes the following issues:

- `string.char()` returns the internal numeric codes of the
  characters, not a single number.
- `string.char()` returns `nil` when values `i` or `j` are
   outside the acceptable range (less than zero and greater than
   the length of the string). It is not documented.

The patch fixes description (synopsis is at the end of the
description) and fixes lower boundary for `n`, with `n` equal
to zero assertion calculates negative number in right part of
expression.

The patch changes function `random_locale()`, so it builds a table
with locales only once on the first execution and then returns
a cached table.

The patcha adds a test for `table.clear()` function that is
present in LuaJIT. Follows up the commit ee032e7
("tests/lapi: add table tests").

The patch updates assertions for values returned by `os.date()`
and `os.time()`, LuaJIT-specific type checks moved under condition.