Added back heuristic-based power-loss testing

The main change here from the previous test framework design is:

1. Powerloss testing remains in-process, speeding up testing.

2. The state of a test, included all powerlosses, is encoded in the
   test id + leb16 encoded powerloss string. This means exhaustive
   testing can be run in CI, but then easily reproduced locally with
   full debugger support.

   For example:

   ./scripts/test.py test_dirs#reentrant_many_dir#10#1248g1g2 --gdb

   Will run the test test_dir, case reentrant_many_dir, permutation #10,
   with powerlosses at 1, 2, 4, 8, 16, and 32 cycles. Dropping into gdb
   if an assert fails.

The changes to the block-device are a work-in-progress for a
lazily-allocated/copy-on-write block device that I'm hoping will keep
exhaustive testing relatively low-cost.

lfs_util.h

@@ -23,6 +23,7 @@
 // System includes
 #include <stdint.h>
 #include <stdbool.h>
+#include <sys/types.h>
 #include <string.h>
 #include <inttypes.h>