Fixed minor things to get CI passing again

- Added caching to Travis install dirs, because otherwise
  pip3 install fails randomly
- Increased size of littlefs-fuse disk because test script has
  a larger footprint now
- Skip a couple of reentrant tests under byte-level writes because
  the tests just take too long and cause Travis to bail due to no
  output for 10m
- Fixed various Valgrind errors
  - Suppressed uninit checks for tests where LFS_BLOCK_ERASE_VALUE == -1.
    In this case rambd goes uninitialized, which is fine for rambd's
    purposes. Note I couldn't figure out how to limit this suppression
    to only the malloc in rambd, this doesn't seem possible with Valgrind.
  - Fixed memory leaks in exhaustion tests
  - Fixed off-by-1 string null-terminator issue in paths tests
- Fixed lfs_file_sync issue caused by revealed by fixing memory leaks
  in exhaustion tests. Getting ENOSPC during a file write puts the file
  in a bad state where littlefs doesn't know how to write it out safely.
  In this case, lfs_file_sync and lfs_file_close return 0 without
  writing out state so that device-side resources can still be cleaned
  up. To recover from ENOSPC, the file needs to be reopened and the
  writes recreated. Not sure if there is a better way to handle this.
- Added some quality-of-life improvements to Valgrind testing
  - Fit Valgrind messages into truncated output when not in verbose mode
  - Turned on origin tracking

tests/test_paths.toml

@@ -247,14 +247,14 @@ code = '''
     lfs_mkdir(&lfs, "coffee/coldcoffee") => 0;
 
     memset(path, 'w', LFS_NAME_MAX+1);
-    path[LFS_NAME_MAX+2] = '\0';
+    path[LFS_NAME_MAX+1] = '\0';
     lfs_mkdir(&lfs, path) => LFS_ERR_NAMETOOLONG;
     lfs_file_open(&lfs, &file, path, LFS_O_WRONLY | LFS_O_CREAT)
             => LFS_ERR_NAMETOOLONG;
 
     memcpy(path, "coffee/", strlen("coffee/"));
     memset(path+strlen("coffee/"), 'w', LFS_NAME_MAX+1);
-    path[strlen("coffee/")+LFS_NAME_MAX+2] = '\0';
+    path[strlen("coffee/")+LFS_NAME_MAX+1] = '\0';
     lfs_mkdir(&lfs, path) => LFS_ERR_NAMETOOLONG;
     lfs_file_open(&lfs, &file, path, LFS_O_WRONLY | LFS_O_CREAT)
             => LFS_ERR_NAMETOOLONG;
@@ -270,7 +270,6 @@ code = '''
     lfs_mkdir(&lfs, "coffee/warmcoffee") => 0;
     lfs_mkdir(&lfs, "coffee/coldcoffee") => 0;
 
-    lfs_mount(&lfs, &cfg) => 0;
     memset(path, 'w', LFS_NAME_MAX);
     path[LFS_NAME_MAX] = '\0';
     lfs_mkdir(&lfs, path) => 0;