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.

Makefile

@@ -110,10 +110,10 @@ tags:
 .PHONY: test-runner
 test-runner: override CFLAGS+=--coverage
 test-runner: $(BUILDDIR)runners/test_runner
+	rm -f $(TEST_GCDA)
 
 .PHONY: test
 test: test-runner
-	rm -f $(TEST_GCDA)
 	./scripts/test.py --runner=$(BUILDDIR)runners/test_runner $(TESTFLAGS)
 
 .PHONY: test-list