tests: posix: env: fix failure in case toolchain with minimal libc

In case of minimal libc we have CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE = 0
by default. Some toolchains uses minimal libc by default (for example
ARC MWDT toolchain). Let's set CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE
explicitly so we can allocate memory for environment variables.

Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>