tests: internal: stream_processor: fixed memory leak

[leonardo-albertovich]

This PR addresses the following leak reported by ASAN :

Direct leak of 65536 byte(s) in 8 object(s) allocated from:
    #0 0x7fb487452d40 in realloc (/lib/x86_64-linux-gnu/libasan.so.4+0xdfd40)
    #1 0x56098a10d88a in msgpack_sbuffer_write /home/runner/work/fluent-bit/fluent-bit/lib/msgpack-c/include/msgpack/sbuffer.h:81
    #2 0x56098a10d3ef in msgpack_pack_array /home/runner/work/fluent-bit/fluent-bit/lib/msgpack-c/include/msgpack/pack_template.h:724
    #3 0x56098a10fd70 in package_results /home/runner/work/fluent-bit/fluent-bit/src/stream_processor/flb_sp.c:1139
    #4 0x56098a101e80 in flb_sp_do_test /home/runner/work/fluent-bit/fluent-bit/tests/internal/stream_processor.c:125
    #5 0x56098a1040d4 in test_window /home/runner/work/fluent-bit/fluent-bit/tests/internal/stream_processor.c:506
    #6 0x56098a0fb83c in acutest_do_run_ /home/runner/work/fluent-bit/fluent-bit/tests/internal/../lib/acutest/acutest.h:1034
    #7 0x56098a0fbc68 in acutest_run_ /home/runner/work/fluent-bit/fluent-bit/tests/internal/../lib/acutest/acutest.h:1130
    #8 0x56098a0fe5d4 in main /home/runner/work/fluent-bit/fluent-bit/tests/internal/../lib/acutest/acutest.h:1769
    #9 0x7fb486bd5082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)