preerase: Expanded preerase testing to include non-0xff erase values

I just noticed we weren't testing preerase with non-0xff ecksums at all!

Added to relevant tests:

  # test with a number of different erase values
  defines.ERASE_VALUE = [0xff, 0x00, -1]

The most important non-0xff value being -1 (noop erases), which should
usually result in fragmented ecksums.

Note this was copied from test_rbyd, where we do something similar to
test non-0xff ecksums in rbyd logs.

tests/test_mount.toml

@@ -336,6 +336,8 @@ defines.SIZE = [
 # REMOUNT=1 => remount with preerase
 # REMOUNT=2 => remount without preerase
 defines.REMOUNT = [0, 1, 2]
+# test with a number of different erase values
+defines.ERASE_VALUE = [0xff, 0x00, -1]
 ifdef = ['LFS3_GBMAP', 'LFS3_REVPERTURB', 'LFS3_PREERASE']
 if = 'GBMAP'
 code = '''