Limited byte-level tests to native testing due to time

Byte-level writes are expensive and not suggested (caches >= 4 bytes
make much more sense), however there are many corner cases with
byte-level writes that can be easy to miss (power-loss leaving single
bytes written to disk).

Unfortunately, byte-level writes mixed with power-loss testing, the
Travis infrastructure, and Arm Thumb instruction set simulation
exceeds the 50-minute budget Travis allocates for jobs.

For now I'm disabling the byte-level tests under Qemu, with the hope that
performance improvements in littlefs will let us turn these tests back
on in the future.

tests/test_dirs.toml

@@ -157,7 +157,6 @@ code = '''
 [[case]] # reentrant many directory creation/rename/removal
 define.N = [5, 11]
 reentrant = true
-if = 'LFS_CACHE_SIZE >= 4' # these just take too long with byte-level writes
 code = '''
     err = lfs_mount(&lfs, &cfg);
     if (err) {
@@ -384,7 +383,6 @@ code = '''
 [[case]] # reentrant file creation/rename/removal
 define.N = [5, 25]
 reentrant = true
-if = 'LFS_CACHE_SIZE >= 4' # these just take too long with byte-level writes
 code = '''
     err = lfs_mount(&lfs, &cfg);
     if (err) {