tests: lib: mem_alloc: Increase malloc heap size for newlib test

The newlib full malloc implementation (i.e. non-nano) requests a
relatively large 4096-byte memory chunk through `_sbrk`, which exceeds
the configured 512-byte heap size.

This commit changes `CONFIG_NEWLIB_LIBC_ALIGNED_HEAP_SIZE` from 512 to
8192 in order to increase the size of the heap memory used by the
newlib malloc function.

For more details, refer to the issue #21167.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>