test/fuzz: fix errors "attempt to compare"

Running of automatically generated Lua programs sometimes failed
with an error "attempt to compare" (5%). The error is caused by
comparison of non-comparable Lua values: string and boolean,
number and function, boolean and number etc. In Lua 5.1 and
LuaJIT 2.1 (which follow the semantics and behaviour of Lua 5.1),
it is not possible to overload Lua metamethods __le and __lt for
these base types. Therefore, an approach with metamethods doesn't
help there.

See an LuaJIT implementation in `src/lj_meta.c:lj_meta_comp()`
and result of comparison values with different types and
metamethods `__le` and `__lt`:

```
$ cat compare.lua
local mt_table = {}
mt_table.__le = function() assert() end
mt_table.__lt = function() assert() end

local res
local str = 'str1'
debug.setmetatable(str, mt_table)

-- __le
res = str <= 10
-- __lt
res = str < 10
$ luajit compare.lua
luajit: compare.lua:10: attempt to compare string with number
stack traceback:
        compare.lua:10: in main chunk
        [C]: at 0x55908425f2e0
$ lua5.1 compare.lua
lua5.1: compare.lua:10: attempt to compare string with number
stack traceback:
        compare.lua:10: in main chunk
        [C]: ?
$ lua5.2 compare.lua
lua5.2: compare.lua:2: assertion failed!
stack traceback:
        [C]: in function 'assert'
        compare.lua:2: in function '__le'
        compare.lua:10: in main chunk
        [C]: in ?
$
```

To fix errors triggered by comparison of non-comparable values
introduced a metamethods `__le` and `__lt` and comparison operators
are wrapped by a Lua function `only_numbers_cmp` that is
implemented in `preamble.lua`. The function perform comparison
only when both values have type 'number', otherwise it returns
`false`.

NO_CHANGELOG=testing
NO_DOC=testing
NO_TEST=testing