Added/fixed tests for noop writes (where bd error can't be trusted)

It's interesting how many ways block devices can show failed writes:
1. prog can error
2. erase can error
3. read can error after writing (ECC failure)
4. prog doesn't error but doesn't write the data correctly
5. erase doesn't error but doesn't erase correctly

Can read fail without an error? Yes, though this appears the same as
prog and erase failing.

These weren't all simulated by testbd since I unintentionally assumed
the block device could always error. Fixed by added additional bad-black
behaviors to testbd.

Note: This also includes a small fix where we can miss bad writes if the
underlying block device contains a valid commit with the exact same
size in the exact same offset.

bd/lfs_testbd.h

@@ -19,14 +19,18 @@ extern "C"
 #endif
 
 
-// Mode determining how "bad blocks" behave during testing. This
-// simulates some real-world circumstances such as writes not
-// going through (noprog), erases not sticking (noerase), and ECC
-// failures (noread).
+// Mode determining how "bad blocks" behave during testing. This simulates
+// some real-world circumstances such as progs not sticking (prog-noop),
+// a readonly disk (erase-noop), and ECC failures (read-error).
+//
+// Not that read-noop is not allowed. Read _must_ return a consistent (but
+// may be arbitrary) value on every read.
 enum lfs_testbd_badblock_behavior {
-    LFS_TESTBD_BADBLOCK_NOPROG  = 0,
-    LFS_TESTBD_BADBLOCK_NOERASE = 1,
-    LFS_TESTBD_BADBLOCK_NOREAD  = 2,
+    LFS_TESTBD_BADBLOCK_PROGERROR,
+    LFS_TESTBD_BADBLOCK_ERASEERROR,
+    LFS_TESTBD_BADBLOCK_READERROR,
+    LFS_TESTBD_BADBLOCK_PROGNOOP,
+    LFS_TESTBD_BADBLOCK_ERASENOOP,
 };
 
 // Type for measuring wear
@@ -82,7 +86,7 @@ typedef struct lfs_testbd {
 /// Block device API ///
 
 // Create a test block device using the geometry in lfs_config
-// 
+//
 // Note that filebd is used if a path is provided, if path is NULL
 // testbd will use rambd which can be much faster.
 int lfs_testbd_create(const struct lfs_config *cfg, const char *path);