It seems we spend a lot of time waiting for `bitcoind` and `lightningd` to
talk to disks. This adds the `TEST_DIR` environment variable, allowing for
example to use `/dev/shm`, or a faster disk than the disk `/tmp` is on, as the
root directory for all test-related files.
Testing this on one of our builder machines cut the time to run the entire
suite under valgrind roughly in half (180-200 seconds vs 440-490 seconds).