tests: net: socket: tcp: reduce number of file descriptors

For some reason, tests/net/socket/tcp/net.socket.tcp.preempt
was failing to link for vmu_rt1170 with these changes, and
for some reason reducing CONFIG_POSIX_MAX_FDS from 20 to 10
resolves that issue.

ld.bfd: scripts/build/gen_kobject_placeholders.py \
  did not reserve enough space for kobject rodata.
Memory region         Used Size  Region Size  %age Used
           FLASH:      287860 B        64 MB      0.43%
             RAM:       86460 B       512 KB     16.49%
           OCRAM:          0 GB       256 KB      0.00%
          OCRAM1:          0 GB       512 KB      0.00%
          OCRAM2:          0 GB       512 KB      0.00%
            ITCM:          0 GB       256 KB      0.00%
            DTCM:       13908 B       256 KB      5.31%
        IDT_LIST:          0 GB         2 KB      0.00%
collect2: error: ld returned 1 exit status

Signed-off-by: Christopher Friedt <cfriedt@meta.com>