Fix append subtree roots

[Oscar-Pepper]

Fixes: #2357

added test "add_subtree_roots". failed on without the subtree root index bugfix in the following way:

$ cargo nextest run -p libtonode-tests add_subtree_roots --no-capture
   Compiling pepper-sync v0.3.0 (/home/oscar/src/zingolib/pepper-sync)
   Compiling zingolib v3.0.1 (/home/oscar/src/zingolib/zingolib)
   Compiling zingolib_testutils v0.1.0 (/home/oscar/src/zingolib/zingolib_testutils)
   Compiling libtonode-tests v0.2.0 (/home/oscar/src/zingolib/libtonode-tests)
    Finished `test` profile [optimized + debuginfo] target(s) in 47.89s
────────────
 Nextest run ID 134ed0c1-82fb-417c-a91a-56cb6e84f7d6 with nextest profile: default
    Starting 1 test across 6 binaries (83 tests skipped)
       START (1/1) libtonode-tests::sync add_subtree_roots

running 1 test

thread 'add_subtree_roots' panicked at libtonode-tests/tests/sync.rs:107:9:
no of sapling shard roots wallet: 1096, no of sapling shard roots server: 1127
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test add_subtree_roots ... FAILED

failures:

failures:
    add_subtree_roots

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 4 filtered out; finished in 8.62s

        FAIL [   8.624s] (1/1) libtonode-tests::sync add_subtree_roots

test passes on this branch.

• also fixes an issue where sync polling states sync is running but sync mode states sync is not running.
• correctly builds libtonode-tests without "darkside-tests" feature which disables necessary RPC calls to test pepper-sync

#2348 will need to merge first to fix ci test failure