fix: test: Add support for small stack size for threads

When running the isc_quota unit test with less than
usual amount of RAM (e.g. in a CI for architectures
with 32 bits of address space), the pthread_create()
function fails with the "Resource temporarily unavailable
(11)" error code.

Add functions to get and set the thread stack size (if requested),
and use these to set the thread stack size to smaller value in the
isc_quota unit test.

Merge branch 'aram/isc-thread-stack-size-small' into 'main'

See merge request isc-projects/bind9!10778