emubd: Renamed lfs_emubd_copy -> lfs_emubd_cpy

For consistency with strcpy/memcpy/lfsr_bd_cpy, though I realize this is
a bit of a weak argument.

runners/test_runner.c

@@ -1609,7 +1609,7 @@ static void powerloss_exhaustive_branch(void *c) {
     }
 
     // create copy-on-write copy
-    int err = lfs_emubd_copy(state->cfg, branch);
+    int err = lfs_emubd_cpy(state->cfg, branch);
     if (err) {
         fprintf(stderr, "error: exhaustive: could not create bd copy\n");
         exit(-1);