Tweaked cache size to temporarily avoid pathological shrub overflows

This will stop being a problem when we actually have btrees, but for now
the fragmentation caused by byte-level syncs was easily enough to
overflow an mdir when cache size is big.

A smaller cache size is also nicer for debugging, since smaller cache
sizes results in data getting flushed to disk earlier, which is easier
to inspect than in-device buffers. And a 16-byte cache still provides
decent test coverage over cache interactions.

---

Also dropped inline_size to block_size/8. I realized while debugging
that opened shrubs take up additional space until we sync, so we need to
expect up to 2 temporary copies of shrubs when writing files.

runners/test_runner.h

@@ -137,8 +137,8 @@ intmax_t test_define(size_t define);
     TEST_DEF(BLOCK_SIZE,         0                                          ) \
     TEST_DEF(BLOCK_COUNT,        DISK_SIZE/BLOCK_SIZE                       ) \
     TEST_DEF(DISK_SIZE,          1024*1024                                  ) \
-    TEST_DEF(CACHE_SIZE,         lfs_max(64, lfs_max(READ_SIZE, PROG_SIZE)) ) \
-    TEST_DEF(INLINE_SIZE,        BLOCK_SIZE/4                               ) \
+    TEST_DEF(CACHE_SIZE,         lfs_max(16, lfs_max(READ_SIZE, PROG_SIZE)) ) \
+    TEST_DEF(INLINE_SIZE,        BLOCK_SIZE/8                               ) \
     TEST_DEF(BUD_SIZE,           BLOCK_SIZE/4                               ) \
     TEST_DEF(LOOKAHEAD_SIZE,     16                                         ) \
     TEST_DEF(BLOCK_CYCLES,       -1                                         ) \