pytest: Allow running tests in a shared memory directory

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).

tests/fixtures.py

@@ -29,7 +29,9 @@ __attempts = {}
 
 @pytest.fixture(scope="session")
 def test_base_dir():
-    directory = tempfile.mkdtemp(prefix='ltests-')
+    d = os.getenv("TEST_DIR", "/tmp")
+
+    directory = tempfile.mkdtemp(prefix='ltests-', dir=d)
     print("Running tests in {}".format(directory))
 
     yield directory