tests: lib: cobs: Fix test setup failure with minimal libc

Set CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=256 for COBS tests to ensure
malloc() succeeds during test fixture allocation in cobs_test_setup().

When using minimal libc, CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE defaults
to 0, causing malloc() to fail and the zassume_not_null(fixture)
assertion to trigger test setup failure.

Fixes test execution with minimal libc configurations.

Signed-off-by: Mohamed Moawad <moawad@synopsys.com>