Previously the functional test could only be run from the electrum root directory. This is a bit inconvenient if we want to run them e.g. from the ${PROJECT_ROOT}/build directory.

Also use os.path.join everywhere instead of concatening strings with unix-style path separators.

from the monorepo root:

pytest electrum/electrumabc/tests/regtest
cd electrum
pytest electrumabc/tests/regtest
cd ../build
pytest ../electrum/electrumabc/tests/regtest