[cmake] Add an override option for tmpdirprefix when running functional tests

Summary: This make it possible to use a ramdisk as storage for running the functional tests, which can bring a nice speedup on some tests.

Test Plan:

mkdir /dev/shm/abc
cmake -GNinja .. -DFUNCTIONAL_TESTS_TMPDIRPREFIX=/dev/shm/abc
ninja check-functional

On my fast ssd machine (so worst case), duration is lowered from 62s to 49s.
On some dev VM with network drive, duration is lowered from 218s to 178s.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17965