test1521: fix compiler warnings

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
9b167fd0.

Closes https://github.com/curl/curl/pull/1611