t/helper/test-json-writer.c: avoid using `strtok()`

t/helper/test-json-writer.c: avoid using `strtok()`

Apply similar treatment as in the previous commit to remove usage of
`strtok()` from the "oidmap" test helper.

Each of the different commands that the "json-writer" helper accepts
pops the next space-delimited token from the current line and interprets
it as a string, integer, or double (with the exception of the very first
token, which is the command itself).

To accommodate this, split the line in place by the space character, and
pass the corresponding string_list to each of the specialized `get_s()`,
`get_i()`, and `get_d()` functions.

`get_i()` and `get_d()` are thin wrappers around `get_s()` that convert
their result into the appropriate type by either calling `strtol()` or
`strtod()`, respectively. In `get_s()`, we mark the token as "consumed"
by incrementing the `consumed_nr` counter, indicating how many tokens we
have read up to that point.

Because each of these functions needs the string-list parts, the number
of tokens consumed, and the line number, these three are wrapped up in
to a struct representing the line state.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>