Took a stab at #12574

Created a `getPath` method which can be used with the `TestingSetup` fixture to create a temp directory. Updated tests using temp directories to use this method.

I tried setting up a `BOOST_GLOBAL_FIXTURE` to create a truly global path for all tests but was getting linker errors when including `boost/test/unit_test.hpp` in `test_bitcoin.cpp`. Even if I had gotten the linking to work, it looks like `make check` invokes the test binary a bunch of times, so it may not have worked anyway.