tests: threads_ext: move min_flash and min_ram parameters to test level

One of the tests is failing since it was split out in 1a633609
triggering an assertion:

ASSERTION FAIL [((((size_t)(16)) << 10) - (((uintptr_t) (&_end)) -
0x20000000)) >= 8192] @
WEST_TOPDIR/zephyr/lib/libc/newlib/libc-hooks.c:139

Fix it by making the min_flash and min_ram parameters up to overall
test level.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>