tests: posix: getopt: hifive1: reduce `MIN_REQUIRED_HEAP_SIZE` to 1024

This commit reduces `CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE` from
2048 to 1024 for the SiFive HiFive1 board.

Otherwise, the test fails on the following assert from Newlib:
memory space available for newlib heap is less than the minimum required
size specified by CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE

Signed-off-by: Patryk Koscik <pkoscik@antmicro.com>