gc: Added more tests over info flags, dropped gc_flags default

Since we dropped lfsr_gc_setflags/setsteps, it was no longer possible to
set gc_flags to zero (perfectly valid and useful for system bringup/
testing things). Supporting gc_flags=0 means it's not possible to
provide a default, but this is probably ok as users need to opt-in to
LFS_GC anyways.

Note that at least gc_steps=0 doesn't make sense, so the default there
is reasonable.

Fixing this also highlighted that gc_flags/steps are no longer mutable,
making the comment in lfs_init out-of-date. Dropping these saves a bit
of lfs_t size, so that's nice.

And then testing also revealed that LFS_GC_CKDATA implying LFS_GC_CKDATA
means it should probably clear the LFS_I_CKMETA flag as well.

---

And here I thought this was going to be just a simple test-writing
exercise!

Code changes:

                   code          stack          ctx
  default before: 37792           2608          620
  default after:  37792 (-0.0%)   2608 (+0.0%)  620 (+0.0%)

  gc before:      37896           2608          768
  gc after:       37848 (-0.1%)   2608 (+0.0%)  760 (-1.0%)

lfs.h

@@ -349,9 +349,6 @@ struct lfs_config {
 
     #ifdef LFS_GC
     // Flags indicating what gc work to do during lfsr_gc calls.
-    //
-    // Defaults to LFS_GC_MKCONSISTENT + LFS_GC_LOOKAHEAD +
-    // LFS_GC_COMPACT when zero.
     uint32_t gc_flags;
     #endif
 
@@ -880,8 +877,6 @@ typedef struct lfs {
 
     #ifdef LFS_GC
     struct {
-        uint32_t flags;
-        lfs_soff_t steps;
         lfsr_traversal_t t;
     } gc;
     #endif