test: increase max fiber slice for box/before_replace

The test fails on osx_debug quite frequently with the following error:

NO_WRAP
> box/before_replace.test.lua                                     [ fail ]
>
> Test failed! Result content mismatch:
> --- box/before_replace.result	Mon Aug  7 10:36:06 2023
> +++ /tmp/t/rejects/box/before_replace.reject	Mon Aug  7 10:42:03 2023
> @@ -899,6 +899,7 @@
>  ...
>  for i = 1,17 do gen_inserts() end
>  ---
> +- error: fiber slice is exceeded
>  ...
>  test_run:cmd('restart server test')
>  s = box.space.test_on_schema_init
> @@ -906,7 +907,7 @@
>  ...
>  s:count()
>  ---
> -- 1
> +- 0
>  ...
>  -- For this test the number of invocations of the before_replace
>  -- trigger during recovery multiplied by the amount of return values
> @@ -921,7 +922,8 @@
>  -- the value is only increased by 1, and this change is visible only in memory.
>  s:get{1}[2] == 1 + 16999 + 17000 + 1 or s:get{1}[2]
>  ---
> -- true
> +- error: '[string "return s:get{1}[2] == 1 + 16999 + 17000 + 1 o..."]:1: attempt to
> +    index a nil value'
>  ...
>  test_run:cmd('switch default')
>  ---
NO_WRAP

Let's try to increase the max fiber slice up to 15 seconds to avoid
that, like we do in other potentially long tests. (The default value
is 1 second.)

NO_DOC=test fix
NO_CHANGELOG=test fix