Added fuzz test attribute

This acts as a marker to indicate a fuzz test. It should reference a
define, usually SEED, that can be randomized to get interesting test
permutations.

This is currently unused, but could lead to some interesting uses such
as time-based fuzz testing. It's also just useful for inspecting the
tests (make test-list).

tests/test_forphans.toml

@@ -6662,6 +6662,7 @@ defines.SIZE = [
     '4*BLOCK_SIZE',
 ]
 defines.SEED = 'range(20)'
+fuzz = 'SEED'
 if = '(SIZE*N)/BLOCK_SIZE <= 16'
 code = '''
     lfs_t lfs;
@@ -7000,6 +7001,7 @@ defines.SIZE = [
     '4*BLOCK_SIZE',
 ]
 defines.SEED = 'range(20)'
+fuzz = 'SEED'
 if = '(SIZE*N)/BLOCK_SIZE <= 16'
 code = '''
     lfs_t lfs;