test1521: fix compiler warnings

[MarcelRaad]

The integer literal 3123123123 doesn't fit into a 32-bit signed
integer, so GCC with 32-bit long warns in C90 mode:
this decimal constant is unsigned only in ISO C90 [enabled by default]
Fix this by using ULONG_MAX, which should fit in any curl_off_t and has
the correct suffix to not issue any warnings.
Also adds the missing CURLOPT_REQUEST_TARGET from commit
9b167fd.

[mention-bot]

@MarcelRaad, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dfandrich and @bagder to be potential reviewers.

[coveralls]

Coverage decreased (-0.03%) to 73.833% when pulling a55449c on MarcelRaad:lib1521_warnings into e5d6aa8 on curl:master.

[bagder]

I'm confused. Isn't ULONG_MAX an even larger value?

[MarcelRaad]

Yes, it is. But is has the UL suffix, so it has the same defined behavior with all language standards. (The alternative would be to add the UL suffix.)
Without a suffix, integers literals that are too big to fit into a signed long become unsigned long in C90, while they become signed long long in C99 and later.

[MarcelRaad]

Thanks for the review!