test: Wait for index sync in wallet_fast_rescan.py #36618

Fixes #36564 by adding the missing wait on the index sync.

Diff to reproduce:

diff --git a/src/util/thread.cpp b/src/util/thread.cpp
index 5b20e807b4..2e988c9eea 100644
--- a/src/util/thread.cpp
+++ b/src/util/thread.cpp
@@ -8,2 +8,3 @@
 #include <util/log.h>
+#include <util/time.h>
 #include <util/threadnames.h>
@@ -17,2 +18,3 @@ void util::TraceThread(std::string_view thread_name, std::function<void()> threa
     util::ThreadRename(std::string{thread_name});
+    UninterruptibleSleep(999ms);
     try {

On failure, the log will say something like:

 - 2026-10-18T05:06:44.451973Z [http.01] [wallet/scan.cpp:142] [ShouldFetchBlock] [scan] Fast rescan: inspect block 200 [4c063a93914ec0cb1604297f2f601dab9b9342bf073e6b80e1c11efd71714e2e] (WARNING: block filter not found!)

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36618
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36618.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36618#author-ralph

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process. A summary of reviews will appear here.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

It seems a bit tedious to find and fix all of those issues by hand. Also, the tests will become overly verbose with all the manual syncs.

The framework should do this by itself, so I pushed that refactor and removed all of the manual syncs in tests.