Allow for using an emulator for the functional test framework

Summary:
This adds the crosscompiling emulator to the test configuration file, and make use of it in the test framework.
This is useful for running the tests with qemu or wine.

Test Plan:
Prerequisite

sudo apt install qemu-user-static

Then:

cmake -GNinja .. \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/LinuxAArch64.cmake \
  -DCMAKE_CROSSCOMPILING_EMULATOR=$(command -v qemu-aarch64-static) \
  -DBUILD_BITCOIN_ZMQ=OFF
QEMU_LD_PREFIX=/usr/aarch64-linux-gnu ninja check-functional

Sanity check for regression:

cmake -GNinja ..
ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5878