Add unit and integration tests for recently changed code

Unit tests (29 new in tests/tests.cpp):
- str::eq() and str::startswith() char* nullptr safety
- str::from() u64/s64/int suffix parsing (K/M/G/T, overflow, invalid)
- num::humanize() byte formatting (0, 512, 1K, 1M, 1G, 1T, non-exact)
- RmdirErr priority error accumulation (ENOTEMPTY/EEXIST override rules)
- HashSet inline-to-heap growth, empty string, and large insertion
- fs::inode algorithm switching (valid/invalid names, passthrough raw ino,
  distinct inodes across algorithms)
- RND::rand64() bounds (range, max, shrink_to_rand_elem)
- Branches::Impl copy and move assignment re-links default_minfreespace

Integration tests (5 new in tests/):
- TEST_policy_lup: LUP policy create and search
- TEST_rmdir_enotempty_priority: rmdir returns ENOTEMPTY for non-empty dirs
- TEST_open_after_unlink: read/write via fd after unlink
- TEST_api_xattr: mergerfs xattr API (fullpath, relpath, basepath, allpaths)
- TEST_movefile_enospc: moveonenospc configuration round-trip

Also: remove xattr nosys checks from TEST_cfg_xattr_modes, reformat
integration test scripts to use temp_dir/cleanup_dir helpers, and
minor parameter alignment changes in fuse_getxattr, fuse_init,
fuse_rename.

src/fuse_init.cpp

@@ -37,7 +37,7 @@
 static
 void
 _want(fuse_conn_info_t *conn_,
-      const int       flag_)
+      const int         flag_)
 {
   conn_->want |= flag_;
 }
@@ -80,7 +80,7 @@ static const char MAX_PAGES_LIMIT_FILEPATH[] = "/proc/sys/fs/fuse/max_pages_limi
 static
 void
 _want_if_capable_max_pages(fuse_conn_info_t *conn_,
-                           Config         &cfg_)
+                           Config           &cfg_)
 {
   std::fstream f;
   u64 max_pages_limit;